fix async handling in SerieScanner and add image_downloader cleanup

- SerieScanner.scan() now detects running event loop and uses create_task()
  when already in async context, avoids RuntimeError
- NFOService.close() now also closes image_downloader to prevent resource leaks
- Add integration tests for TMDBClient lifecycle management

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/core/services/nfo_service.py

@@ -784,6 +784,7 @@ class NFOService:
     async def close(self):
         """Clean up resources."""
         await self.tmdb_client.close()
+        await self.image_downloader.close()
     
     async def create_minimal_nfo(
         self,