- Add SQLAlchemy models for User, Translation, ApiKey, UsageLog, PaymentHistory
- Add database connection management with PostgreSQL/SQLite support
- Add repository layer for CRUD operations
- Add Alembic migration setup with initial migration
- Update auth_service to automatically use database when DATABASE_URL is set
- Update docker-compose.yml with PostgreSQL service and Redis (non-optional)
- Add database migration script (scripts/migrate_to_db.py)
- Update .env.example with database configuration