Fix: media index build never starts (blocking Jellyfin dependency)

The build endpoint had Depends(get_jellyfin_client) and Depends(get_user_id)
which executed BEFORE the function body. If Jellyfin was unreachable, these
raised HTTPException(503), the function never ran, and the worker was never
started. The frontend mutation had no onError handler, so the failure was
completely silent — the button briefly showed 'Building...' then reverted
to 'Build index' with zero feedback.

Backend fix: removed the Jellyfin dependencies from post_build_index.
The worker subprocess resolves its own Jellyfin connection via
_resolve_jellyfin(service_id) — the endpoint just needs to start the
worker process. The libraries count starts at 0 and gets updated by
the worker once it connects.

Frontend fix: added onError to useBuildIndex that invalidates the status
query (so the UI reflects the non-building state). MediaTab now displays
the build error inline: 'Build failed: <message>' next to the button.

283 backend tests pass (updated build test for new no-dependency flow);
128 frontend tests pass; ruff/eslint clean.

backend/tests/test_api.py

@@ -363,7 +363,7 @@ class TestMediaIndexApi:
                 assert data["status"] == "started"
                 assert data["build_running"] is True
                 assert data["build_stage"] == "queued"
-                assert data["build_libraries_total"] == len(mock_jellyfin.libraries.return_value)
+                assert data["build_libraries_total"] == 0
                 assert data["build_pid"] == 4321
                 start_worker.assert_called_once()
             finally: