feat: add automatic balance timestamp migration mechanism

- Add migration system to convert Unix timestamps to datetime strings
- Integrate migration into FastAPI lifespan for automatic startup execution
- Update balance persistence to use consistent ISO datetime format
- Fix mixed timestamp types causing API parsing issues
- Add comprehensive error handling and progress logging
- Successfully migrated 7522 balance records to consistent format

leggend/main.py

@@ -24,6 +24,17 @@ async def lifespan(app: FastAPI):
         logger.error(f"Failed to load configuration: {e}")
         raise
 
+    # Run database migrations
+    try:
+        from leggend.services.database_service import DatabaseService
+
+        db_service = DatabaseService()
+        await db_service.run_migrations_if_needed()
+        logger.info("Database migrations completed")
+    except Exception as e:
+        logger.error(f"Database migration failed: {e}")
+        raise
+
     # Start background scheduler
     scheduler.start()
     logger.info("Background scheduler started")