Backend: add backups + authentik service types, absorb jellyseerr (Slice 1)

New service types:
- backups: BackupsConfig(ingestion_label), no secrets, summary widget kind.
  Modeled as a service so it can be named/multi-instanced like others.
- authentik: AuthentikConfig(base_url, timeout_seconds), api_token secret
  (required). Directory source for the upcoming Users tab.

Jellyseerr absorption:
- JellyfinConfig gains optional jellyseerr_url + jellyseerr_api_key fields.
- integrations/jellyseerr.py deleted; registry entry removed.
- clients/jellyseerr.py stays (JellyseerrClient still used by enrichment).
- One-time idempotent migration in settings_store.ensure_defaults():
  jellyseerr service rows merge into a paired Jellyfin (exactly-one merges;
  multiple picks first unpaired; none/all-paired drops with a logged
  warning). The api_key is decrypted from secrets before moving to config.

Registry is now 8 types: alertmanager, authentik, backups, grafana,
jellyfin, nextcloud, prometheus, ssh_tasks.

Tests: registry count updated to 8, jellyseerr-absent assertion, new-type
definition assertions, and migration tests (single-jellyfin merge, no-
jellyfin drop, idempotency). 256 backend tests pass; ruff clean.

Refs openspec/changes/services-as-hub-ia/ (spec R6, tasks slice 1).

backend/src/media_library_viewer_api/integrations/jellyfin.py

@@ -13,11 +13,20 @@ from media_library_viewer_api.integrations.base import (
 
 
 class JellyfinConfig(ServiceConfigBase):
-    """Non-secret Jellyfin connection config."""
+    """Non-secret Jellyfin connection config.
+
+    The optional ``jellyseerr_url`` / ``jellyseerr_api_key`` fields carry the
+    paired Jellyseerr companion config, absorbed from the former standalone
+    ``jellyseerr`` service type (see OpenSpec change ``services-as-hub-ia``).
+    When both are set, the Jellyfin service page renders a Requests tab backed
+    by Jellyseerr.
+    """
 
     base_url: ServiceBaseUrl
     user_id: str = ""
     timeout_seconds: int = 10
+    jellyseerr_url: str = ""
+    jellyseerr_api_key: str = ""
 
 
 class JellyfinActivityWidgetConfig(WidgetConfigBase):