feat(db): migrate to PostgreSQL 16 standalone

- docker-compose.prod.yml: add postgres:16-alpine service with health check,
  dedicated prod_leopost_net, backup volume mount, connection pool
- requirements.txt: add psycopg2-binary==2.9.9
- database.py: remove SQLite-specific run_migrations(), add PG pool_size/
  max_overflow/pool_pre_ping, keep sqlite compat for dev
- main.py: remove run_migrations call, rely on create_all for PG
- scripts/migrate_sqlite_to_pg.py: one-shot data migration script

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

backend/app/main.py

@@ -17,7 +17,7 @@ from fastapi.staticfiles import StaticFiles
 
 from .auth import hash_password
 from .config import settings
-from .database import Base, SessionLocal, engine, run_migrations
+from .database import Base, SessionLocal, engine
 from .models import User
 from .routers.admin import router as admin_router
 from .routers.auth import router as auth_router
@@ -79,10 +79,7 @@ async def lifespan(app: FastAPI):
     data_dir = Path("./data")
     data_dir.mkdir(parents=True, exist_ok=True)
 
-    # Run migrations FIRST (add new columns to existing tables)
-    run_migrations(engine)
-
-    # Create tables (for new tables like subscription_codes)
+    # Create all tables (PostgreSQL: idempotent, safe to run on every startup)
     Base.metadata.create_all(bind=engine)
 
     # Create or update admin user