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/widgets/builtin.py

@@ -0,0 +1,68 @@
+"""Built-in, service-less widget kinds.
+
+These widgets do not talk to an external service and therefore have no
+``service_id``. They are kept out of the service registry (which models
+configurable external services) and live here as a small closed set.
+
+Currently: ``backups`` (reads the internal backup tables) and ``static``
+(plain text/markdown).
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from media_library_viewer_api.integrations.base import WidgetKind
+
+BUILTIN_WIDGET_KINDS: dict[str, WidgetKind] = {
+    "backups": WidgetKind(
+        kind="backups",
+        name="Backups",
+        description="Backup job summary and active alerts.",
+        config_schema={"type": "object", "properties": {}, "required": []},
+        default_config={},
+        refresh_interval_ms=60_000,
+    ),
+    "static": WidgetKind(
+        kind="static",
+        name="Static text",
+        description="Plain text or markdown note.",
+        config_schema={
+            "type": "object",
+            "properties": {"text": {"type": "string", "description": "Text or markdown content"}},
+            "required": ["text"],
+        },
+        default_config={"text": ""},
+        refresh_interval_ms=0,
+    ),
+}
+
+
+def get_builtin_widget_kind(kind: str) -> WidgetKind | None:
+    return BUILTIN_WIDGET_KINDS.get(kind)
+
+
+def is_builtin_kind(kind: str) -> bool:
+    return kind in BUILTIN_WIDGET_KINDS
+
+
+def builtin_widget_kind_models() -> dict[str, type]:
+    """Pydantic widget-config models for built-in kinds (validated manually).
+
+    Backups has no user fields; static validates ``text``.
+    """
+    from pydantic import BaseModel, Field
+
+    class StaticConfig(BaseModel):
+        text: str = Field(default="")
+
+    return {"static": StaticConfig}
+
+
+def validate_builtin_config(kind: str, config: dict[str, Any]) -> dict[str, Any]:
+    """Validate (lightly) a built-in widget config and return the cleaned dict."""
+    models = builtin_widget_kind_models()
+    model_cls = models.get(kind)
+    if model_cls is None:
+        return dict(config or {})
+    return model_cls.model_validate(config or {}).model_dump(exclude_none=True)