Store events and config in SQLite

Replace the JSON-file event and config stores with a SQLite database
(modernc.org/sqlite, pure-Go so the static build keeps CGO_ENABLED=0).
Last-write-wins now rides on the upsert's WHERE clause rather than a
Go-side map compare; the sync protocol and HTTP handlers are unchanged.

On first start the server auto-imports any legacy events.json/config.json
sitting in the data dir, renaming them to *.imported. The -data flag now
points at puppy.db; photos still live on the filesystem alongside it.

README.md

@@ -15,6 +15,10 @@ source-of-truth and sync between devices.
   client POSTs its full event list to `/api/events/sync`. The server merges
   it with its own copy using last-write-wins on `updatedAt` and returns the
   merged set.
+- The server keeps its copy in a SQLite database (`puppy.db`); events and the
+  shared profile are separate tables, and last-write-wins is enforced by the
+  upsert itself. On first start it auto-imports any legacy `events.json` /
+  `config.json` sitting alongside it, renaming them to `*.imported`.
 - Service worker bypasses cache for `/api/*` so writes always hit the server
   when online; static assets are still cached for offline use.
 - The puppy's name and birthday are a shared profile stored on the host
@@ -35,7 +39,8 @@ puppy-tracker/
 ├── module.nix        # systemd unit, StateDirectory, hardening
 ├── server/
 │   ├── go.mod
-│   └── main.go       # JSON-file store, LWW sync, static file serving
+│   ├── go.sum
+│   └── main.go       # SQLite store, LWW sync, static file serving
 └── src/              # the web app
     ├── index.html
     ├── app.js
@@ -52,7 +57,7 @@ nix run                       # http://localhost:8080, data in $XDG_DATA_HOME/pu
 PUPPY_ADDR=:9000 nix run      # custom port
 
 # Hot-iterate (data in /tmp):
-nix develop -c sh -c 'cd server && go run . -static ../src -data /tmp/puppy-events.json'
+nix develop -c sh -c 'cd server && go run . -static ../src -data /tmp/puppy.db'
 ```
 
 ## Use it on NixOS
@@ -81,8 +86,9 @@ In your system flake:
 }
 ```
 
-The server runs as a `DynamicUser` systemd unit. Data is stored at
-`/var/lib/puppy-tracker/events.json` via `StateDirectory`.
+The server runs as a `DynamicUser` systemd unit. Data is stored in a SQLite
+database at `/var/lib/puppy-tracker/puppy.db` via `StateDirectory` (with photos
+alongside it under `photos/`).
 
 ## Notes