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/requirements.txt

@@ -1,6 +1,7 @@
 fastapi==0.115.0
 uvicorn[standard]==0.30.6
 sqlalchemy==2.0.35
+psycopg2-binary==2.9.9
 pydantic==2.9.2
 pydantic-settings==2.5.2
 python-jose[cryptography]==3.3.0