Make retry handlers static methods

Convert handle_network_failure and handle_download_failure from instance methods to static methods. Hardcode retry params (max_retries, delays) instead of using instance state. Improves testability and removes implicit dependencies.

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

src/server/utils/dependencies.py

@@ -163,7 +163,7 @@ def get_series_app() -> SeriesApp:
                         ),
                     )
 
-            _series_app = SeriesApp(anime_dir, db_lookup=_make_db_lookup())
+            _series_app = SeriesApp(anime_dir)
         except Exception as e:
             raise HTTPException(
                 status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,