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/package.json

@@ -20,7 +20,9 @@
   "dependencies": {
     "axios": "^1.6.0",
     "bcrypt": "^5.1.1",
+    "connect-pg-simple": "^10.0.0",
     "connect-sqlite3": "^0.9.16",
+    "pg": "^8.13.0",
     "cors": "^2.8.5",
     "express": "^4.18.2",
     "express-rate-limit": "^8.5.2",