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/jellyseerr.py

@@ -0,0 +1,32 @@
+"""Jellyseerr service definition.
+
+Jellyseerr is a companion to Jellyfin (request management). It is modeled as its
+own service type so multiple Jellyseerr instances are supported independently of
+Jellyfin. It provides no dashboard widgets today.
+"""
+
+from __future__ import annotations
+
+from media_library_viewer_api.integrations.base import (
+    SecretField,
+    ServiceConfigBase,
+    ServiceDefinition,
+)
+
+
+class JellyseerrConfig(ServiceConfigBase):
+    """Non-secret Jellyseerr connection config."""
+
+    base_url: str
+
+
+DEFINITION = ServiceDefinition(
+    service_type="jellyseerr",
+    name="Jellyseerr",
+    description="Request management companion to Jellyfin.",
+    config_model=JellyseerrConfig,
+    secret_fields=[
+        SecretField(key="api_key", label="API key", required=True),
+    ],
+    widget_kinds=[],
+)