Download history refactor (#700)

- Much simpler handling of downloads in the activity sidebar, and
improved storage, persistence and UI behavior.
- Replace `ActivityService` with direct storage on
`DownloadHistoryService` and `download_requests` and removes the
activity_log/activity_dismissals tables
- Simplify no-auth mode by removing the fake user row pattern, handled
internally
- Add local download fallback so history entries can still serve files
after tasks leave the queue
- Downloads, requests and history are now entirely persistent between
updates / restarts, and correctly tied to each user.

tests/e2e/test_api.py

@@ -207,14 +207,6 @@ class TestQueueEndpoint:
         else:
             assert isinstance(data, list)
 
-    def test_clear_queue(self, api_client: APIClient, download_tracker: DownloadTracker):
-        """Test clearing the queue."""
-        resp = api_client.delete("/api/queue/clear")
-
-        # Should succeed (may be 200 or 204)
-        assert resp.status_code in [200, 204]
-
-
 @pytest.mark.e2e
 class TestSettingsEndpoint:
     """Tests for settings endpoints."""