feat(widgets): rebind widgets to the service registry

PR 2 of 4 for the runtime service registry change.

- dashboard_widgets gains service_id + widget_kind columns (legacy
  addon_id/widget_type kept but unused).
- Source adapters take (service: ServiceRecord | None, widget_kind, config).
  SERVICE_ADAPTERS keyed by service_type; BUILTIN_ADAPTERS for backups/static.
- Backups and static stay as service-less built-ins (service_id nullable),
  exposed via GET /api/widgets/builtin.
- SSH task adapter resolves the task + instance, runs over SSH, and appends a
  service_task_runs history row on success/failure/timeout/error.
- Retire widgets/registry.py; widget metadata now comes from the integrations
  registry + widgets/builtin. Remove /api/widgets/types and /api/widgets/sources.
- Stop default widget seeding (fresh install = empty dashboard).
- Rewrite widget tests around the service-bound + built-in model (26 tests).

Backend-only breaking change; frontend is reconciled in Slice 3. Build/lint
stay green; pytest 222 passed.

backend/src/media_library_viewer_api/integrations/base.py

@@ -59,6 +59,7 @@ class WidgetKind:
     config_schema: dict[str, Any]
     default_config: dict[str, Any] = field(default_factory=dict)
     refresh_interval_ms: int = 0
+    config_model: type[WidgetConfigBase] | None = None
 
 
 @dataclass(frozen=True)
@@ -108,6 +109,7 @@ def widget_kind(
         config_schema=schema,
         default_config=dict(default_config or {}),
         refresh_interval_ms=refresh_interval_ms,
+        config_model=model_cls,
     )