Add self-service account deletion

Settings → Delete account removes the signed-in account and everything it
owns. DELETE /api/me re-checks the password (guarding an unattended session),
then wipes the user's events, config, sessions and user row in one transaction
and removes their photos/<user_id>/ directory. The client clears the account's
local cache and returns to the login screen.

Bumps the service-worker cache so clients pick up the new UI.

Verified: wrong password is rejected (401, data intact); correct password
returns 204, invalidates the session, drops all rows to zero and removes the
photo dir; the email can be re-registered afterwards. Confirmed end to end in
a headless-browser run of the Settings → delete flow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/auth.go

@@ -381,3 +381,62 @@ func (a *Auth) handleMe(w http.ResponseWriter, r *http.Request) {
 	}
 	writeUser(w, u)
 }
+
+// checkPassword reports whether password matches the stored hash for userID.
+func (a *Auth) checkPassword(userID, password string) bool {
+	var hash string
+	if err := a.db.QueryRow(`SELECT password FROM users WHERE id = ?`, userID).Scan(&hash); err != nil {
+		return false
+	}
+	return bcrypt.CompareHashAndPassword([]byte(hash), []byte(password)) == nil
+}
+
+// deleteAccount removes a user and everything owned by them: events, profile,
+// sessions, the user row, and their photo directory. The table wipes run in one
+// transaction; photos are best-effort afterwards (orphaned files are harmless).
+func (a *Auth) deleteAccount(userID string) error {
+	tx, err := a.db.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+	for _, q := range []string{
+		`DELETE FROM events WHERE user_id = ?`,
+		`DELETE FROM config WHERE user_id = ?`,
+		`DELETE FROM sessions WHERE user_id = ?`,
+		`DELETE FROM users WHERE id = ?`,
+	} {
+		if _, err := tx.Exec(q, userID); err != nil {
+			return err
+		}
+	}
+	if err := tx.Commit(); err != nil {
+		return err
+	}
+	if err := os.RemoveAll(filepath.Join(a.photosDir, userID)); err != nil {
+		log.Printf("delete photos for %s: %v", userID, err)
+	}
+	return nil
+}
+
+// handleDeleteAccount deletes the caller's own account after re-checking their
+// password (guards against an unattended session). Wrapped in requireUser.
+func (a *Auth) handleDeleteAccount(w http.ResponseWriter, r *http.Request) {
+	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
+	}
+	uid := userID(r)
+	if !a.checkPassword(uid, c.Password) {
+		http.Error(w, "invalid password", http.StatusUnauthorized)
+		return
+	}
+	if err := a.deleteAccount(uid); err != nil {
+		log.Printf("delete account %s: %v", uid, err)
+		http.Error(w, "server error", http.StatusInternalServerError)
+		return
+	}
+	a.clearCookie(w)
+	w.WriteHeader(http.StatusNoContent)
+}