fix: add missing data_service singleton instance export

- Added data_service = DataService() at module level
- Fixed ImportError in cache_warming_service
- Required for application startup

src/services/data_service.py

@@ -43,4 +43,7 @@ class DataService:
     def clear_cache(self) -> None:
         """Clear service cache."""
         self._cache.clear()
-        self._last_updated = datetime.now()
+        self._last_updated = datetime.now()
+
+# Create singleton instance
+data_service = DataService()