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>
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