feat(auth): migrate user store from SQLite to PostgreSQL

Users were wiped on container recreation because they shared the
backend_data volume with ephemeral pharmacy/medicine data. Postgres
gets its own named volume (postgres_data) that survives deploys.

- Add postgres:16-alpine service + postgres_data volume to compose
- Add pg + connect-pg-simple deps
- userDbGet/userDbRun helpers: PG when PG_URL set, SQLite fallback
- Sessions use connect-pg-simple when pgPool available
- create-admin.js: PG-aware, SQLite fallback for local dev
- push_subscriptions.user_id: plain INTEGER (cross-DB FK dropped)
- Tests unchanged — no PG_URL in test env = SQLite fallback

Set PG_PASSWORD in server .env before next deploy.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/.env.example

@@ -6,6 +6,10 @@ REDIS_HOST=localhost
 REDIS_PORT=6379
 REDIS_PASSWORD=
 
+# PostgreSQL for user accounts (leave unset to fallback to SQLite — dev/test only)
+PG_URL=postgresql://farmafinder:change-me@localhost:5432/farmafinder
+PG_PASSWORD=change-me
+
 # Web Push (VAPID). Generate with:
 #   node -e "import('web-push').then(w => console.log(w.default.generateVAPIDKeys()))"
 VAPID_PUBLIC_KEY=