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

main.py

@@ -204,6 +204,18 @@ async def lifespan(app: FastAPI):
     # Startup
     logger.info("Starting Document Translation API...")
     config.ensure_directories()
+    
+    # Initialize database
+    try:
+        from database.connection import init_db, check_db_connection
+        init_db()
+        if check_db_connection():
+            logger.info("✅ Database connection verified")
+        else:
+            logger.warning("⚠️  Database connection check failed")
+    except Exception as e:
+        logger.warning(f"⚠️  Database initialization skipped: {e}")
+    
     await cleanup_manager.start()
     logger.info("API ready to accept requests")