feat: Add PostgreSQL database infrastructure

- 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

.env.example

@@ -72,9 +72,16 @@ MAX_REQUEST_SIZE_MB=100
 # Request timeout in seconds
 REQUEST_TIMEOUT_SECONDS=300
 
-# ============== Database (Production) ==============
+# ============== Database ==============
 # PostgreSQL connection string (recommended for production)
-# DATABASE_URL=postgresql://user:password@localhost:5432/translate_db
+# Format: postgresql://user:password@host:port/database
+# DATABASE_URL=postgresql://translate_user:secure_password@localhost:5432/translate_db
+
+# SQLite path (used when DATABASE_URL is not set - for development)
+SQLITE_PATH=data/translate.db
+
+# Enable SQL query logging (for debugging)
+DATABASE_ECHO=false
 
 # Redis for sessions and caching (recommended for production)
 # REDIS_URL=redis://localhost:6379/0