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/models/widgets.py

@@ -1,8 +1,18 @@
-"""Pydantic models for the dashboard widget system."""
+"""Pydantic models for the dashboard widget system.
+
+Widgets are either:
+* **service-bound** — reference a ``service_id`` and a ``widget_kind`` declared
+  by that service's definition (Grafana link, Prometheus metric, Jellyfin
+  activity, SSH task output); or
+* **built-in** — ``service_id`` is null and ``widget_kind`` is one of the
+  service-less kinds (backups, static).
+"""
+
+from __future__ import annotations
 
 from typing import Any
 
-from pydantic import BaseModel, Field, field_validator
+from pydantic import BaseModel, Field, field_validator, model_validator
 
 FORBIDDEN_CONFIG_KEYS = {
     "password",
@@ -47,8 +57,8 @@ def _validate_config_keys(config: dict[str, Any]) -> dict[str, Any]:
 class _WidgetInstanceBase(BaseModel):
     """Shared fields between input and output widget models."""
 
-    addon_id: str
-    widget_type: str
+    service_id: str | None = None
+    widget_kind: str = Field(..., min_length=1)
     title: str = Field(..., min_length=1)
     config: dict[str, Any] = Field(default_factory=dict)
     enabled: bool = True
@@ -59,6 +69,13 @@ class _WidgetInstanceBase(BaseModel):
     def reject_credential_keys(cls, value: dict[str, Any]) -> dict[str, Any]:
         return _validate_config_keys(value or {})
 
+    @model_validator(mode="after")
+    def _validate_kind(self) -> "_WidgetInstanceBase":
+        # The kind must be non-empty (Field enforces it); service_id may be None
+        # for built-ins. Deeper validation happens in the router against the
+        # service definition / built-in registry.
+        return self
+
 
 class WidgetInstanceInput(_WidgetInstanceBase):
     """Payload for creating or updating a widget instance."""
@@ -74,22 +91,21 @@ class WidgetInstance(_WidgetInstanceBase):
     updated_at: int
 
 
-class WidgetTypeInfo(BaseModel):
-    """Metadata about a built-in widget type."""
+class BuiltinWidgetKindInfo(BaseModel):
+    """Metadata about a built-in (service-less) widget kind."""
 
-    addon_id: str
-    widget_type: str
+    kind: str
     name: str
     description: str
-    source_type: str
     config_schema: dict[str, Any]
+    default_config: dict[str, Any]
+    refresh_interval_ms: int
 
 
 class WidgetDataResponse(BaseModel):
     """Response from the per-widget data endpoint."""
 
     widget_id: str
-    widget_type: str
     data: dict[str, Any] | None = None
     error: str | None = None
     fetched_at: int