feat(service-storage-harness): slice 3 — migrate MediaIndex onto harness (scoped replace_items, +service_id)

Register MediaIndex as a harness concern with ALTER TABLE migration to add
service_id column (idempotent). Scope replace_items by service_id (FIXES latent
global-clear bug where building for one Jellyfin wiped another's rows). Scope
query by service_id (empty-string = all rows, backward-compat). Thread
service_id through build_media_index + worker + query_media router. New
regression test proves scoped replace preserves other services' rows.

Backend: 321 pytest pass, ruff clean. Frontend: build green.

backend/src/media_library_viewer_api/services/service_data.py

@@ -143,6 +143,10 @@ def get_service_data_harness() -> ServiceDataHarness:
         from media_library_viewer_api.services.qbittorrent_store import QBITTORRENT_CONCERN
 
         _HARNESS.register(QBITTORRENT_CONCERN)
+
+        from media_library_viewer_api.services.media_index_impl import MEDIA_INDEX_CONCERN
+
+        _HARNESS.register(MEDIA_INDEX_CONCERN)
         _HARNESS.run_migrations()
     return _HARNESS