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>

docker-compose.yml

@@ -3,6 +3,16 @@ services:
     image: redis:alpine
     restart: unless-stopped
 
+  postgres:
+    image: postgres:16-alpine
+    restart: unless-stopped
+    environment:
+      POSTGRES_DB: farmafinder
+      POSTGRES_USER: farmafinder
+      POSTGRES_PASSWORD: ${PG_PASSWORD:-change-me-in-production}
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+
   backend:
     image: git.hacecalor.net/ichitux/farmafinder-backend:latest
     build:
@@ -21,10 +31,12 @@ services:
       REDIS_PASSWORD: ${REDIS_PASSWORD:-}
       DATABASE_PATH: /app/data/database.sqlite
       FARMACIAS_WEBHOOK_URL: ${FARMACIAS_WEBHOOK_URL:-}
+      PG_URL: postgresql://farmafinder:${PG_PASSWORD:-change-me-in-production}@postgres:5432/farmafinder
     volumes:
       - backend_data:/app/data
     depends_on:
       - redis
+      - postgres
 
   frontend:
     image: git.hacecalor.net/ichitux/farmafinder-frontend:latest
@@ -37,3 +49,4 @@ services:
 
 volumes:
   backend_data:
+  postgres_data: