Add accounts and multi-tenancy

Every event, profile and photo is now scoped to a signed-in account, so
separate people can track separate puppies on one server.

Server:
- users + sessions tables; bcrypt passwords; random session tokens stored
  hashed and set as an HttpOnly cookie. Middleware gates /api/* behind a
  valid session.
- register/login/logout/me endpoints. Registration requires a shared invite
  code (-invite-code / PUPPY_INVITE_CODE); empty disables it.
- events, config and photos are keyed by user_id; the sync upsert guards
  against cross-user overwrites and reads are scoped, so accounts are isolated.
  Photos live under photos/<user_id>/ and are only served to their owner.
- in-place schema migration adds user_id and reshapes config; legacy
  single-tenant data (including imported events.json) is parked ownerless and
  adopted by the first account to register.

Client:
- login/register gate in front of the app; the tracker only boots once the
  session check resolves. localStorage is namespaced per user.
- 401s bounce back to login; an offline reload falls back to the last cached
  session so offline-first still works. Logout clears the session and reloads.

Deployment:
- module.nix gains inviteCodeFile (secret via EnvironmentFile) and
  secureCookies options.

Verified end to end (curl + a headless-browser run of the auth flow):
isolation between accounts, invite enforcement, first-user adoption, photo
ownership, and session persistence across reload.

server/auth.go

@@ -0,0 +1,383 @@
+package main
+
+import (
+	"context"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/subtle"
+	"database/sql"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"golang.org/x/crypto/bcrypt"
+)
+
+const (
+	sessionCookie   = "puppy_session"
+	sessionValidity = 30 * 24 * time.Hour
+)
+
+// ctxKey is an unexported type so our context values can't collide with any
+// set elsewhere.
+type ctxKey int
+
+const userIDKey ctxKey = 0
+
+// User is the public shape returned to clients — never the password hash.
+type User struct {
+	ID    string `json:"id"`
+	Email string `json:"email"`
+}
+
+// Auth owns everything account-related: the users/sessions tables, the shared
+// invite code required to register, and whether session cookies are marked
+// Secure (on behind TLS/a proxy). photosDir is needed so the first account can
+// adopt legacy flat-layout photos.
+type Auth struct {
+	db         *sql.DB
+	inviteCode string
+	secure     bool
+	photosDir  string
+}
+
+func newAuth(db *sql.DB, inviteCode string, secure bool, photosDir string) *Auth {
+	return &Auth{db: db, inviteCode: inviteCode, secure: secure, photosDir: photosDir}
+}
+
+// ---------- users & sessions ----------
+
+func newID() string {
+	b := make([]byte, 16)
+	if _, err := rand.Read(b); err != nil {
+		panic(err) // crypto/rand failing is unrecoverable
+	}
+	// RFC-4122-ish v4 layout; good enough as an opaque unique id.
+	b[6] = (b[6] & 0x0f) | 0x40
+	b[8] = (b[8] & 0x3f) | 0x80
+	return hex.EncodeToString(b[0:4]) + "-" + hex.EncodeToString(b[4:6]) + "-" +
+		hex.EncodeToString(b[6:8]) + "-" + hex.EncodeToString(b[8:10]) + "-" +
+		hex.EncodeToString(b[10:16])
+}
+
+// hashToken stores only the hash of a session token, so a leaked database can't
+// be used to impersonate live sessions.
+func hashToken(raw string) string {
+	sum := sha256.Sum256([]byte(raw))
+	return hex.EncodeToString(sum[:])
+}
+
+func (a *Auth) userCount() (int, error) {
+	var n int
+	err := a.db.QueryRow(`SELECT COUNT(*) FROM users`).Scan(&n)
+	return n, err
+}
+
+var errEmailTaken = errors.New("email already registered")
+
+func (a *Auth) createUser(email, password string) (User, error) {
+	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		return User{}, err
+	}
+	u := User{ID: newID(), Email: email}
+	_, err = a.db.Exec(
+		`INSERT INTO users (id, email, password, created) VALUES (?, ?, ?, ?)`,
+		u.ID, email, string(hash), time.Now().UnixMilli())
+	if err != nil {
+		if strings.Contains(err.Error(), "UNIQUE") {
+			return User{}, errEmailTaken
+		}
+		return User{}, err
+	}
+	return u, nil
+}
+
+// verify returns the user for the given credentials, or ok=false if the email
+// is unknown or the password is wrong (indistinguishable to the caller).
+func (a *Auth) verify(email, password string) (User, bool) {
+	var u User
+	var hash string
+	err := a.db.QueryRow(
+		`SELECT id, email, password FROM users WHERE email = ? COLLATE NOCASE`, email,
+	).Scan(&u.ID, &u.Email, &hash)
+	if err != nil {
+		if !errors.Is(err, sql.ErrNoRows) {
+			log.Printf("verify: %v", err)
+		}
+		return User{}, false
+	}
+	if bcrypt.CompareHashAndPassword([]byte(hash), []byte(password)) != nil {
+		return User{}, false
+	}
+	return u, true
+}
+
+// startSession mints a token, stores its hash, and returns the raw token for
+// the cookie.
+func (a *Auth) startSession(userID string) (string, error) {
+	raw := make([]byte, 32)
+	if _, err := rand.Read(raw); err != nil {
+		return "", err
+	}
+	token := hex.EncodeToString(raw)
+	now := time.Now()
+	_, err := a.db.Exec(
+		`INSERT INTO sessions (token, user_id, created, expires) VALUES (?, ?, ?, ?)`,
+		hashToken(token), userID, now.UnixMilli(), now.Add(sessionValidity).UnixMilli())
+	if err != nil {
+		return "", err
+	}
+	return token, nil
+}
+
+// userForToken resolves a raw cookie token to a user id, honouring expiry.
+func (a *Auth) userForToken(token string) (string, bool) {
+	if token == "" {
+		return "", false
+	}
+	var userID string
+	var expires int64
+	err := a.db.QueryRow(
+		`SELECT user_id, expires FROM sessions WHERE token = ?`, hashToken(token),
+	).Scan(&userID, &expires)
+	if err != nil || time.Now().UnixMilli() > expires {
+		return "", false
+	}
+	return userID, true
+}
+
+func (a *Auth) endSession(token string) {
+	if token == "" {
+		return
+	}
+	if _, err := a.db.Exec(`DELETE FROM sessions WHERE token = ?`, hashToken(token)); err != nil {
+		log.Printf("endSession: %v", err)
+	}
+}
+
+// adopt gives every ownerless row (legacy single-tenant data) to userID, and
+// moves legacy flat-layout photos into that user's photo directory. Called once,
+// when the very first account registers.
+func (a *Auth) adopt(userID string) error {
+	if _, err := a.db.Exec(`UPDATE events SET user_id = ? WHERE user_id = ''`, userID); err != nil {
+		return err
+	}
+	if _, err := a.db.Exec(`UPDATE config SET user_id = ? WHERE user_id = ''`, userID); err != nil {
+		return err
+	}
+	return a.adoptPhotos(userID)
+}
+
+// adoptPhotos moves any *.jpg sitting directly in photosDir (the pre-accounts
+// flat layout) into photosDir/<userID>/.
+func (a *Auth) adoptPhotos(userID string) error {
+	entries, err := os.ReadDir(a.photosDir)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil
+		}
+		return err
+	}
+	dstDir := filepath.Join(a.photosDir, userID)
+	for _, e := range entries {
+		if e.IsDir() || !strings.HasSuffix(e.Name(), ".jpg") {
+			continue
+		}
+		if err := os.MkdirAll(dstDir, 0o755); err != nil {
+			return err
+		}
+		if err := os.Rename(
+			filepath.Join(a.photosDir, e.Name()),
+			filepath.Join(dstDir, e.Name()),
+		); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// ---------- cookies & middleware ----------
+
+func (a *Auth) setCookie(w http.ResponseWriter, token string) {
+	http.SetCookie(w, &http.Cookie{
+		Name:     sessionCookie,
+		Value:    token,
+		Path:     "/",
+		HttpOnly: true,
+		Secure:   a.secure,
+		SameSite: http.SameSiteLaxMode,
+		Expires:  time.Now().Add(sessionValidity),
+	})
+}
+
+func (a *Auth) clearCookie(w http.ResponseWriter) {
+	http.SetCookie(w, &http.Cookie{
+		Name:     sessionCookie,
+		Value:    "",
+		Path:     "/",
+		HttpOnly: true,
+		Secure:   a.secure,
+		SameSite: http.SameSiteLaxMode,
+		MaxAge:   -1,
+	})
+}
+
+func cookieToken(r *http.Request) string {
+	c, err := r.Cookie(sessionCookie)
+	if err != nil {
+		return ""
+	}
+	return c.Value
+}
+
+// requireUser wraps a handler so it only runs for an authenticated request,
+// stashing the user id in the context. Unauthenticated calls get a 401 that the
+// client uses as its cue to show the login screen.
+func (a *Auth) requireUser(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		userID, ok := a.userForToken(cookieToken(r))
+		if !ok {
+			http.Error(w, "unauthorized", http.StatusUnauthorized)
+			return
+		}
+		next(w, r.WithContext(context.WithValue(r.Context(), userIDKey, userID)))
+	}
+}
+
+// userID returns the authenticated user's id; only valid inside a requireUser
+// handler.
+func userID(r *http.Request) string {
+	id, _ := r.Context().Value(userIDKey).(string)
+	return id
+}
+
+// ---------- handlers ----------
+
+type credentials struct {
+	Email    string `json:"email"`
+	Password string `json:"password"`
+	Invite   string `json:"invite"`
+}
+
+func writeUser(w http.ResponseWriter, u User) {
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("Cache-Control", "no-store")
+	_ = json.NewEncoder(w).Encode(u)
+}
+
+func (a *Auth) handleRegister(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	if a.inviteCode == "" {
+		http.Error(w, "registration disabled", http.StatusForbidden)
+		return
+	}
+	var c credentials
+	if err := json.NewDecoder(io.LimitReader(r.Body, 1<<16)).Decode(&c); err != nil {
+		http.Error(w, "bad json", http.StatusBadRequest)
+		return
+	}
+	// Constant-time compare so a wrong invite code can't be timed out.
+	if subtle.ConstantTimeCompare([]byte(c.Invite), []byte(a.inviteCode)) != 1 {
+		http.Error(w, "invalid invite code", http.StatusForbidden)
+		return
+	}
+	email := strings.TrimSpace(strings.ToLower(c.Email))
+	if !strings.Contains(email, "@") || len(email) > 200 {
+		http.Error(w, "invalid email", http.StatusBadRequest)
+		return
+	}
+	if len(c.Password) < 8 || len(c.Password) > 200 {
+		http.Error(w, "password must be at least 8 characters", http.StatusBadRequest)
+		return
+	}
+
+	// Whether this is the first account decides adoption of legacy data. Check
+	// before insert; the users table has no other writer during registration.
+	first, err := a.userCount()
+	if err != nil {
+		http.Error(w, "server error", http.StatusInternalServerError)
+		return
+	}
+	u, err := a.createUser(email, c.Password)
+	if errors.Is(err, errEmailTaken) {
+		http.Error(w, "email already registered", http.StatusConflict)
+		return
+	}
+	if err != nil {
+		log.Printf("register: %v", err)
+		http.Error(w, "server error", http.StatusInternalServerError)
+		return
+	}
+	if first == 0 {
+		if err := a.adopt(u.ID); err != nil {
+			log.Printf("adopt legacy data: %v", err)
+			// Non-fatal: the account exists; legacy data just stays ownerless.
+		}
+	}
+	a.issue(w, u)
+}
+
+func (a *Auth) handleLogin(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	var c credentials
+	if err := json.NewDecoder(io.LimitReader(r.Body, 1<<16)).Decode(&c); err != nil {
+		http.Error(w, "bad json", http.StatusBadRequest)
+		return
+	}
+	u, ok := a.verify(strings.TrimSpace(strings.ToLower(c.Email)), c.Password)
+	if !ok {
+		http.Error(w, "invalid email or password", http.StatusUnauthorized)
+		return
+	}
+	a.issue(w, u)
+}
+
+// issue starts a session, sets the cookie, and returns the user.
+func (a *Auth) issue(w http.ResponseWriter, u User) {
+	token, err := a.startSession(u.ID)
+	if err != nil {
+		log.Printf("start session: %v", err)
+		http.Error(w, "server error", http.StatusInternalServerError)
+		return
+	}
+	a.setCookie(w, token)
+	writeUser(w, u)
+}
+
+func (a *Auth) handleLogout(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	a.endSession(cookieToken(r))
+	a.clearCookie(w)
+	w.WriteHeader(http.StatusNoContent)
+}
+
+// handleMe reports the current account. Wrapped in requireUser, so reaching it
+// means the session is valid.
+func (a *Auth) handleMe(w http.ResponseWriter, r *http.Request) {
+	var u User
+	err := a.db.QueryRow(
+		`SELECT id, email FROM users WHERE id = ?`, userID(r),
+	).Scan(&u.ID, &u.Email)
+	if err != nil {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+	writeUser(w, u)
+}

server/go.mod

@@ -2,7 +2,10 @@ module puppy-tracker
 
 go 1.25.0
 
-require modernc.org/sqlite v1.53.0
+require (
+	golang.org/x/crypto v0.54.0
+	modernc.org/sqlite v1.53.0
+)
 
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
@@ -10,7 +13,7 @@ require (
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
-	golang.org/x/sys v0.44.0 // indirect
+	golang.org/x/sys v0.47.0 // indirect
 	modernc.org/libc v1.73.4 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect

server/go.sum

@@ -12,13 +12,15 @@ github.com/ncruces/go-strftime v1.0.0 h1:HMFp8mLCTPp341M/ZnA4qaf7ZlsbTc+miZjCLOF
 github.com/ncruces/go-strftime v1.0.0/go.mod h1:Fwc5htZGVVkseilnfgOVb9mKy6w1naJmn9CehxcKcls=
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94icq4NjY3clb7Lk8O1qJ8BdBEF8z0ibU0rE=
 github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
+golang.org/x/crypto v0.54.0 h1:YLIA59K4fiNzHzjnZt2tUJQjQtUWfWbeHBqKtk3eScw=
+golang.org/x/crypto v0.54.0/go.mod h1:KWL8ny2AZdGR2cWmzeHrp2azQPGogOv+HeQaVEXC2dk=
 golang.org/x/mod v0.36.0 h1:JJjpVx6myfUsUdAzZuOSTTmRE0PfZeNWzzvKrP7amb4=
 golang.org/x/mod v0.36.0/go.mod h1:moc6ELqsWcOw5Ef3xVprK5ul/MvtVvkIXLziUOICjUQ=
 golang.org/x/sync v0.20.0 h1:e0PTpb7pjO8GAtTs2dQ6jYa5BWYlMuX047Dco/pItO4=
 golang.org/x/sync v0.20.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
 golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.44.0 h1:ildZl3J4uzeKP07r2F++Op7E9B29JRUy+a27EibtBTQ=
-golang.org/x/sys v0.44.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
+golang.org/x/sys v0.47.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
 golang.org/x/tools v0.45.0 h1:18qN3FAooORvApf5XjCXgsuayZOEtXf6JK18I3+ONa8=
 golang.org/x/tools v0.45.0/go.mod h1:LuUGqqaXcXMEFEruIVJVm5mgDD8vww/z/SR1gQ4uE/0=
 modernc.org/cc/v4 v4.28.4 h1:Hd/4Es+MBj+/7hSdZaisNyu6bv3V0Dp2MdllyfqaH+c=

server/main.go

@@ -54,12 +54,12 @@ func newConfigStore(db *sql.DB) *ConfigStore {
 	return &ConfigStore{db: db}
 }
 
-func (cs *ConfigStore) get() Config {
+func (cs *ConfigStore) get(userID string) Config {
 	var c Config
-	// The profile lives in a single row (id = 1). A missing row is the
-	// pre-configuration state, so a zero-value Config is the right answer.
+	// One profile row per user. A missing row is the pre-configuration state,
+	// so a zero-value Config is the right answer.
 	err := cs.db.QueryRow(
-		`SELECT name, birthday, updated FROM config WHERE id = 1`,
+		`SELECT name, birthday, updated FROM config WHERE user_id = ?`, userID,
 	).Scan(&c.Name, &c.Birthday, &c.UpdatedAt)
 	if err != nil && !errors.Is(err, sql.ErrNoRows) {
 		log.Printf("config get: %v", err)
@@ -67,22 +67,22 @@ func (cs *ConfigStore) get() Config {
 	return c
 }
 
-// merge applies an incoming config with last-write-wins by UpdatedAt and
-// returns the resulting stored config (which the caller sends back).
-func (cs *ConfigStore) merge(in Config) (Config, error) {
+// merge applies an incoming config for one user with last-write-wins by
+// UpdatedAt and returns the resulting stored config (which the caller sends back).
+func (cs *ConfigStore) merge(userID string, in Config) (Config, error) {
 	// The upsert's WHERE clause enforces last-write-wins: the incoming row only
 	// replaces the stored one when it is strictly newer.
 	_, err := cs.db.Exec(`
-		INSERT INTO config (id, name, birthday, updated)
-		VALUES (1, ?, ?, ?)
-		ON CONFLICT(id) DO UPDATE SET
+		INSERT INTO config (user_id, name, birthday, updated)
+		VALUES (?, ?, ?, ?)
+		ON CONFLICT(user_id) DO UPDATE SET
 			name = excluded.name, birthday = excluded.birthday, updated = excluded.updated
 		WHERE excluded.updated > config.updated`,
-		in.Name, in.Birthday, in.UpdatedAt)
+		userID, in.Name, in.Birthday, in.UpdatedAt)
 	if err != nil {
 		return Config{}, err
 	}
-	return cs.get(), nil
+	return cs.get(userID), nil
 }
 
 type Store struct {
@@ -93,9 +93,10 @@ func newStore(db *sql.DB) *Store {
 	return &Store{db: db}
 }
 
-// sync merges client events into the store using last-write-wins by UpdatedAt,
-// then returns the full merged set (including tombstones, which must propagate).
-func (s *Store) sync(client []Event) ([]Event, error) {
+// sync merges one user's client events into the store using last-write-wins by
+// UpdatedAt, then returns that user's full merged set (tombstones included, as
+// they must propagate).
+func (s *Store) sync(userID string, client []Event) ([]Event, error) {
 	tx, err := s.db.Begin()
 	if err != nil {
 		return nil, err
@@ -104,14 +105,18 @@ func (s *Store) sync(client []Event) ([]Event, error) {
 
 	// The WHERE clause on the upsert is the last-write-wins rule: an incoming
 	// event only overwrites the stored one when its updatedAt is strictly newer.
+	// The `events.user_id = excluded.user_id` guard means one user can never
+	// clobber another's row even if a client forges a colliding event ID —
+	// the row stays put and, because reads are scoped, stays invisible to them.
 	stmt, err := tx.Prepare(`
-		INSERT INTO events (id, type, at, note, photo_id, weight, updated, deleted)
-		VALUES (?, ?, ?, ?, ?, ?, ?, ?)
+		INSERT INTO events (id, type, at, note, photo_id, weight, updated, deleted, user_id)
+		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
 		ON CONFLICT(id) DO UPDATE SET
 			type = excluded.type, at = excluded.at, note = excluded.note,
 			photo_id = excluded.photo_id, weight = excluded.weight,
 			updated = excluded.updated, deleted = excluded.deleted
-		WHERE excluded.updated > events.updated`)
+		WHERE excluded.updated > events.updated
+		  AND events.user_id = excluded.user_id`)
 	if err != nil {
 		return nil, err
 	}
@@ -122,7 +127,7 @@ func (s *Store) sync(client []Event) ([]Event, error) {
 			continue
 		}
 		if _, err := stmt.Exec(
-			ce.ID, ce.Type, ce.At, ce.Note, ce.PhotoID, ce.Weight, ce.UpdatedAt, ce.Deleted,
+			ce.ID, ce.Type, ce.At, ce.Note, ce.PhotoID, ce.Weight, ce.UpdatedAt, ce.Deleted, userID,
 		); err != nil {
 			return nil, err
 		}
@@ -130,13 +135,14 @@ func (s *Store) sync(client []Event) ([]Event, error) {
 	if err := tx.Commit(); err != nil {
 		return nil, err
 	}
-	return s.all()
+	return s.all(userID)
 }
 
-// all returns every stored event, tombstones included.
-func (s *Store) all() ([]Event, error) {
+// all returns one user's events, tombstones included.
+func (s *Store) all(userID string) ([]Event, error) {
 	rows, err := s.db.Query(
-		`SELECT id, type, at, note, photo_id, weight, updated, deleted FROM events`)
+		`SELECT id, type, at, note, photo_id, weight, updated, deleted
+		   FROM events WHERE user_id = ?`, userID)
 	if err != nil {
 		return nil, err
 	}
@@ -175,6 +181,9 @@ func openDB(path string) (*sql.DB, error) {
 			return nil, err
 		}
 	}
+	// Fresh-install schema. Every row is scoped to a user_id; the empty string
+	// is the "ownerless" bucket that legacy single-tenant data lands in until
+	// the first account adopts it (see Auth.adopt).
 	schema := `
 		CREATE TABLE IF NOT EXISTS events (
 			id       TEXT PRIMARY KEY,
@@ -184,21 +193,105 @@ func openDB(path string) (*sql.DB, error) {
 			photo_id TEXT NOT NULL DEFAULT '',
 			weight   REAL NOT NULL DEFAULT 0,
 			updated  INTEGER NOT NULL DEFAULT 0,
-			deleted  INTEGER NOT NULL DEFAULT 0
+			deleted  INTEGER NOT NULL DEFAULT 0,
+			user_id  TEXT NOT NULL DEFAULT ''
 		);
+		CREATE INDEX IF NOT EXISTS idx_events_user ON events(user_id);
 		CREATE TABLE IF NOT EXISTS config (
-			id       INTEGER PRIMARY KEY CHECK (id = 1),
+			user_id  TEXT PRIMARY KEY,
 			name     TEXT NOT NULL DEFAULT '',
 			birthday TEXT NOT NULL DEFAULT '',
 			updated  INTEGER NOT NULL DEFAULT 0
+		);
+		CREATE TABLE IF NOT EXISTS users (
+			id       TEXT PRIMARY KEY,
+			email    TEXT NOT NULL UNIQUE COLLATE NOCASE,
+			password TEXT NOT NULL,
+			created  INTEGER NOT NULL
+		);
+		CREATE TABLE IF NOT EXISTS sessions (
+			token   TEXT PRIMARY KEY,
+			user_id TEXT NOT NULL,
+			created INTEGER NOT NULL,
+			expires INTEGER NOT NULL
 		);`
 	if _, err := db.Exec(schema); err != nil {
 		db.Close()
 		return nil, err
 	}
+	if err := migrateSchema(db); err != nil {
+		db.Close()
+		return nil, err
+	}
 	return db, nil
 }
 
+// migrateSchema upgrades a single-tenant database (from before accounts existed)
+// in place: it adds events.user_id and rewrites the config table from its
+// old single-row (id = 1) shape to one keyed by user_id. Pre-accounts data ends
+// up ownerless (user_id = ”), ready for the first account to adopt. It is a
+// no-op on a fresh DB, where openDB already created the current schema.
+func migrateSchema(db *sql.DB) error {
+	has, err := columnExists(db, "events", "user_id")
+	if err != nil {
+		return err
+	}
+	if !has {
+		if _, err := db.Exec(`ALTER TABLE events ADD COLUMN user_id TEXT NOT NULL DEFAULT ''`); err != nil {
+			return err
+		}
+		if _, err := db.Exec(`CREATE INDEX IF NOT EXISTS idx_events_user ON events(user_id)`); err != nil {
+			return err
+		}
+	}
+	oldConfig, err := columnExists(db, "config", "id")
+	if err != nil {
+		return err
+	}
+	if oldConfig {
+		// Rebuild config keyed by user_id, moving the lone id=1 row into the
+		// ownerless bucket.
+		stmts := []string{
+			`ALTER TABLE config RENAME TO config_old`,
+			`CREATE TABLE config (
+				user_id  TEXT PRIMARY KEY,
+				name     TEXT NOT NULL DEFAULT '',
+				birthday TEXT NOT NULL DEFAULT '',
+				updated  INTEGER NOT NULL DEFAULT 0
+			)`,
+			`INSERT INTO config (user_id, name, birthday, updated)
+				SELECT '', name, birthday, updated FROM config_old WHERE id = 1`,
+			`DROP TABLE config_old`,
+		}
+		for _, s := range stmts {
+			if _, err := db.Exec(s); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// columnExists reports whether the given table has a column of the given name.
+// A missing table reports false (no error), which is what fresh installs want.
+func columnExists(db *sql.DB, table, col string) (bool, error) {
+	rows, err := db.Query(`SELECT name FROM pragma_table_info(?)`, table)
+	if err != nil {
+		return false, err
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var name string
+		if err := rows.Scan(&name); err != nil {
+			return false, err
+		}
+		if name == col {
+			return true, nil
+		}
+	}
+	return false, rows.Err()
+}
+
 // migrateJSON imports a pre-SQLite events.json / config.json sitting in dataDir
 // into an otherwise-empty database, then renames each file to *.imported so the
 // import runs exactly once. It is a no-op when the DB already holds data or the
@@ -232,8 +325,10 @@ func importEvents(db *sql.DB, path string) error {
 	if err != nil && !errors.Is(err, io.EOF) {
 		return err
 	}
+	// Imported as ownerless (user_id = ""); the first account to register adopts
+	// them. Mirrors how in-place schema migration parks legacy rows.
 	store := newStore(db)
-	if _, err := store.sync(evs); err != nil {
+	if _, err := store.sync("", evs); err != nil {
 		return err
 	}
 	log.Printf("migrated %d events from %s", len(evs), path)
@@ -266,7 +361,8 @@ func importConfig(db *sql.DB, path string) error {
 		// Nothing meaningful to import; leave config empty.
 		return os.Rename(path, path+".imported")
 	}
-	if _, err := newConfigStore(db).merge(c); err != nil {
+	// Ownerless until the first account adopts it (see importEvents).
+	if _, err := newConfigStore(db).merge("", c); err != nil {
 		return err
 	}
 	log.Printf("migrated config from %s", path)
@@ -292,6 +388,10 @@ func main() {
 	addr := flag.String("addr", ":8080", "listen address (e.g. :8080 or 0.0.0.0:8080)")
 	dataPath := flag.String("data", "puppy.db", "path to SQLite database file")
 	staticDir := flag.String("static", "", "directory of static files to serve")
+	inviteCode := flag.String("invite-code", os.Getenv("PUPPY_INVITE_CODE"),
+		"shared secret required to register (env PUPPY_INVITE_CODE); empty disables registration")
+	secureCookies := flag.Bool("secure-cookies", false,
+		"mark session cookies Secure (enable when served over HTTPS / behind a TLS proxy)")
 	flag.Parse()
 
 	db, err := openDB(*dataPath)
@@ -313,9 +413,19 @@ func main() {
 		log.Fatalf("mkdir photos: %v", err)
 	}
 
+	auth := newAuth(db, *inviteCode, *secureCookies, photosDir)
+	if *inviteCode == "" {
+		log.Print("WARNING: no invite code set — registration is disabled (set -invite-code / PUPPY_INVITE_CODE)")
+	}
+
 	mux := http.NewServeMux()
 
-	mux.HandleFunc("/api/events/sync", func(w http.ResponseWriter, r *http.Request) {
+	mux.HandleFunc("/api/register", auth.handleRegister)
+	mux.HandleFunc("/api/login", auth.handleLogin)
+	mux.HandleFunc("/api/logout", auth.handleLogout)
+	mux.HandleFunc("/api/me", auth.requireUser(auth.handleMe))
+
+	mux.HandleFunc("/api/events/sync", auth.requireUser(func(w http.ResponseWriter, r *http.Request) {
 		if r.Method != http.MethodPost {
 			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 			return
@@ -325,7 +435,7 @@ func main() {
 			http.Error(w, "bad json: "+err.Error(), http.StatusBadRequest)
 			return
 		}
-		merged, err := store.sync(req.Events)
+		merged, err := store.sync(userID(r), req.Events)
 		if err != nil {
 			log.Printf("sync: %v", err)
 			http.Error(w, "server error", http.StatusInternalServerError)
@@ -337,11 +447,11 @@ func main() {
 			Events:    merged,
 			ServerNow: time.Now().UnixMilli(),
 		})
-	})
+	}))
 
-	// GET  /api/config — return the shared puppy profile.
+	// GET  /api/config — return the caller's puppy profile.
 	// PUT  /api/config — update it (last-write-wins by updatedAt).
-	mux.HandleFunc("/api/config", func(w http.ResponseWriter, r *http.Request) {
+	mux.HandleFunc("/api/config", auth.requireUser(func(w http.ResponseWriter, r *http.Request) {
 		writeConfig := func(c Config) {
 			w.Header().Set("Content-Type", "application/json")
 			w.Header().Set("Cache-Control", "no-store")
@@ -349,7 +459,7 @@ func main() {
 		}
 		switch r.Method {
 		case http.MethodGet:
-			writeConfig(configStore.get())
+			writeConfig(configStore.get(userID(r)))
 		case http.MethodPut, http.MethodPost:
 			var in Config
 			if err := json.NewDecoder(io.LimitReader(r.Body, 1<<16)).Decode(&in); err != nil {
@@ -364,7 +474,7 @@ func main() {
 				http.Error(w, "invalid birthday (want YYYY-MM-DD)", http.StatusBadRequest)
 				return
 			}
-			merged, err := configStore.merge(in)
+			merged, err := configStore.merge(userID(r), in)
 			if err != nil {
 				log.Printf("config save: %v", err)
 				http.Error(w, "server error", http.StatusInternalServerError)
@@ -374,7 +484,7 @@ func main() {
 		default:
 			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 		}
-	})
+	}))
 
 	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
 		w.Write([]byte("ok"))
@@ -383,7 +493,7 @@ func main() {
 	// POST /api/photos — multipart upload with form fields `id` (UUID) and
 	// `file` (JPEG). The client generates the ID so the event referencing
 	// the photo can be written before the upload round-trips.
-	mux.HandleFunc("/api/photos", func(w http.ResponseWriter, r *http.Request) {
+	mux.HandleFunc("/api/photos", auth.requireUser(func(w http.ResponseWriter, r *http.Request) {
 		if r.Method != http.MethodPost {
 			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 			return
@@ -405,7 +515,14 @@ func main() {
 		}
 		defer file.Close()
 
-		dstPath := filepath.Join(photosDir, id+".jpg")
+		// Photos live under the owner's directory so a photo can only ever be
+		// read back by the account that uploaded it.
+		userDir := filepath.Join(photosDir, userID(r))
+		if err := os.MkdirAll(userDir, 0o755); err != nil {
+			http.Error(w, "mkdir: "+err.Error(), http.StatusInternalServerError)
+			return
+		}
+		dstPath := filepath.Join(userDir, id+".jpg")
 		tmp := dstPath + ".tmp"
 		dst, err := os.Create(tmp)
 		if err != nil {
@@ -432,11 +549,12 @@ func main() {
 		w.Header().Set("Content-Type", "application/json")
 		w.Header().Set("Cache-Control", "no-store")
 		_ = json.NewEncoder(w).Encode(map[string]string{"id": id})
-	})
+	}))
 
-	// GET /api/photos/<id> — serves the JPEG. Photos are immutable per ID
-	// so we mark them as long-lived; both browser and SW can cache freely.
-	mux.HandleFunc("/api/photos/", func(w http.ResponseWriter, r *http.Request) {
+	// GET /api/photos/<id> — serves the caller's own JPEG. Photos are immutable
+	// per ID so we mark them as long-lived; the private cache keeps them per
+	// user. Serving only from the caller's directory makes ownership implicit.
+	mux.HandleFunc("/api/photos/", auth.requireUser(func(w http.ResponseWriter, r *http.Request) {
 		if r.Method != http.MethodGet && r.Method != http.MethodHead {
 			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 			return
@@ -446,7 +564,7 @@ func main() {
 			http.Error(w, "invalid id", http.StatusBadRequest)
 			return
 		}
-		path := filepath.Join(photosDir, id+".jpg")
+		path := filepath.Join(photosDir, userID(r), id+".jpg")
 		f, err := os.Open(path)
 		if err != nil {
 			http.NotFound(w, r)
@@ -459,9 +577,9 @@ func main() {
 			return
 		}
 		w.Header().Set("Content-Type", "image/jpeg")
-		w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
+		w.Header().Set("Cache-Control", "private, max-age=31536000, immutable")
 		http.ServeContent(w, r, path, stat.ModTime(), f)
-	})
+	}))
 
 	if *staticDir != "" {
 		fileServer := http.FileServer(http.Dir(*staticDir))