feat(service-storage-harness): slice 1 — harness + qbit store + client + integration

ServiceDataHarness (services/service_data.py): lifecycle-only registry of
per-concern storage — DB provisioning, idempotent migrations (ALTER TABLE
duplicate-column-name caught per-statement), cascade_delete(service_id).
QbittorrentSampleStore: append/window/prune (MAX_SAMPLES=120) in
qbittorrent.db. QbittorrentClient: cookie-login Web API client (403 re-login,
/sync/maindata). Integration registered with 3 widget kinds (totals/active/
speed). Harness initialized in main.py lifespan.

Backend: 314 pytest pass (21 new), ruff clean.

backend/src/media_library_viewer_api/main.py

@@ -52,6 +52,12 @@ async def lifespan(app: FastAPI):
         get_settings_store().ensure_defaults()
     except Exception:
         logger.exception("Failed to seed default settings during startup")
+    try:
+        from media_library_viewer_api.services.service_data import get_service_data_harness
+
+        get_service_data_harness()
+    except Exception:
+        logger.exception("Failed to initialize service data harness during startup")
     mail_queue = get_mail_queue()
     backup_poller = get_backup_poller()
     mail_queue.start()