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/tests/test_services.py

@@ -57,7 +57,7 @@ def client(tmp_path):
 # ---------------------------------------------------------------------------
 
 
-def test_registry_contains_seven_service_types():
+def test_registry_contains_eight_service_types():
     assert set(SERVICE_DEFINITIONS) == {
         "prometheus",
         "alertmanager",
@@ -66,6 +66,7 @@ def test_registry_contains_seven_service_types():
         "ssh_tasks",
         "backups",
         "authentik",
+        "qbittorrent",
     }
 
 
@@ -173,6 +174,7 @@ def test_list_service_types(client):
         "jellyfin",
         "nextcloud",
         "prometheus",
+        "qbittorrent",
         "ssh_tasks",
     }