Reusable widgets: reference widgets across dashboards + detach to clone

Widgets configured on one dashboard (e.g., a Grafana service's Overview)
can now be live-referenced on other dashboards. Editing the widget config
updates it everywhere it's referenced. References can be detached into
independent clones.

Backend: new widget_references table (dashboard_scope, widget_id,
sort_order) with ON DELETE CASCADE. CRUD methods + 4 endpoints:
GET/POST /api/widgets/references, DELETE /api/widgets/references/{id},
POST /api/widgets/references/{id}/detach (clones the widget into a
standalone instance, then removes the reference).

Frontend: WidgetConfigDialog gains a dashboardScope prop. When set
(the main Dashboard passes 'main'), the dialog shows:
- Owned + referenced widgets in a combined list, with a link badge on
  references.
- 'Add existing widget' picker: searchable list of ALL widget instances
  not already on this dashboard. Click to create a reference.
- Detach button on references: clones the widget (service_id=NULL) and
  removes the reference.
- Delete on a reference removes the REFERENCE (not the original widget).

Dashboard renders referenced widgets alongside owned widgets.

Detaching a service-bound widget clones it with service_id=NULL — the
clone may need re-binding to a service to render correctly. Named
dashboards don't pass dashboardScope yet (pinned-links-only); when they
gain widget support, the backend already handles any scope string.

282 backend tests pass (+2 reference lifecycle); 127 frontend tests
pass; ruff/eslint/tsc/vite all green.

backend/src/media_library_viewer_api/routers/widgets.py

@@ -12,6 +12,7 @@ import time
 from typing import Any
 
 from fastapi import APIRouter, Depends, HTTPException, status
+from pydantic import BaseModel
 
 from media_library_viewer_api.dependencies import get_settings_store
 from media_library_viewer_api.integrations.base import validate_config
@@ -34,6 +35,15 @@ from media_library_viewer_api.widgets.sources import (
     get_service_adapter,
 )
 
+
+class WidgetReferenceCreate(BaseModel):
+    """Payload for creating a widget reference (live-link)."""
+
+    dashboard_scope: str
+    widget_id: str
+    sort_order: int = 0
+
+
 router = APIRouter(prefix="/api/widgets", tags=["widgets"])
 
 logger = logging.getLogger(__name__)
@@ -221,3 +231,52 @@ async def fetch_data(
         error=data.get("error"),
         fetched_at=int(time.time()),
     ).model_dump()
+
+
+# ---------------------------------------------------------------------------
+# Widget references (live-link widgets across dashboards)
+# ---------------------------------------------------------------------------
+
+
+@router.get("/references")
+def list_references(
+    dashboard_scope: str,
+    store: SettingsStore = Depends(get_settings_store),
+) -> list[dict[str, Any]]:
+    """List widget references for a dashboard scope."""
+    return store.list_widget_references(dashboard_scope)
+
+
+@router.post("/references", status_code=status.HTTP_201_CREATED)
+def create_reference(
+    body: WidgetReferenceCreate,
+    store: SettingsStore = Depends(get_settings_store),
+) -> dict[str, Any]:
+    """Create a widget reference (live-link) on a dashboard."""
+    try:
+        return store.create_widget_reference(body.dashboard_scope, body.widget_id, body.sort_order)
+    except ValueError as exc:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=str(exc)) from exc
+
+
+@router.delete("/references/{reference_id}")
+def delete_reference(
+    reference_id: str,
+    store: SettingsStore = Depends(get_settings_store),
+) -> dict[str, str]:
+    """Remove a widget reference from a dashboard."""
+    store.delete_widget_reference(reference_id)
+    return {"status": "deleted"}
+
+
+@router.post("/references/{reference_id}/detach")
+def detach_reference(
+    reference_id: str,
+    store: SettingsStore = Depends(get_settings_store),
+) -> dict[str, Any]:
+    """Clone the referenced widget into a standalone instance and remove the reference."""
+    try:
+        cloned = store.detach_widget_reference(reference_id, "")
+        return WidgetInstance(**cloned).model_dump()
+    except ValueError as exc:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=str(exc)) from exc