feat(services): resolve Jellyfin/Jellyseerr from the service registry (backend)

Slice 4b backend half. Jellyfin and Jellyseerr clients are now resolved from
service instances instead of machine-level app config.

- Add jellyseerr service definition (6 service types total); add user_id to
  the Jellyfin service config.
- dependencies.py: jellyfin_service_id query param + _service_record
  (decrypt-on-read); get_jellyfin_client / get_jellyseerr_client / get_user_id
  resolve against the service registry (first enabled instance as fallback).
- SSH/Files transport (get_ssh_client) unchanged; still uses machine_id.
- Update service-registry tests for 6 types.

Selection model: split params — ?jellyfin_service_id= for Jellyfin/Jellyseerr,
?machine_id= for SSH/Files. Frontend threading follows in the next PR.

Verification: backend ruff clean, pytest 222 passed; frontend green (unchanged).

backend/src/media_library_viewer_api/integrations/registry.py

@@ -9,6 +9,7 @@ from __future__ import annotations
 from media_library_viewer_api.integrations.base import ServiceDefinition, WidgetKind
 from media_library_viewer_api.integrations.grafana import DEFINITION as GRAFANA
 from media_library_viewer_api.integrations.jellyfin import DEFINITION as JELLYFIN
+from media_library_viewer_api.integrations.jellyseerr import DEFINITION as JELLYSEERR
 from media_library_viewer_api.integrations.nextcloud import DEFINITION as NEXTCLOUD
 from media_library_viewer_api.integrations.prometheus import DEFINITION as PROMETHEUS
 from media_library_viewer_api.integrations.ssh_tasks import DEFINITION as SSH_TASKS
@@ -17,6 +18,7 @@ SERVICE_DEFINITIONS: dict[str, ServiceDefinition] = {
     GRAFANA.service_type: GRAFANA,
     PROMETHEUS.service_type: PROMETHEUS,
     JELLYFIN.service_type: JELLYFIN,
+    JELLYSEERR.service_type: JELLYSEERR,
     NEXTCLOUD.service_type: NEXTCLOUD,
     SSH_TASKS.service_type: SSH_TASKS,
 }