refactor: encapsulate task clearing behind TaskManager.clear_all()

backend/utils/tasks.py

@@ -76,6 +76,11 @@ class TaskManager:
         """Cancel/dismiss a download task (removes it from active list)."""
         return self._active_downloads.pop(model_name, None) is not None
 
+    def clear_all(self) -> None:
+        """Clear all download and generation tasks."""
+        self._active_downloads.clear()
+        self._active_generations.clear()
+
     def is_download_active(self, model_name: str) -> bool:
         """Check if a download is active."""
         return model_name in self._active_downloads