feat(widgets): add backend source adapters and per-widget data endpoint

PR 2 of 4 for configurable dashboard widgets.

- Add grafana_url and prometheus_url settings (config.py + compose/env).
- Create WidgetSource protocol and adapters for jellyfin, backups, grafana,
  prometheus, ssh_task, and static sources.
- Add GET /api/widgets/instances/{id}/data endpoint.
- Extract shared dashboard helpers into domain/dashboard.py so widgets and
  the dashboard router reuse the same logic.
- Add adapter and data-endpoint tests.
- Update apply-progress.md.

Verification: ruff clean; backend pytest 200 passed; frontend lint/build green.

backend/src/media_library_viewer_api/routers/widgets.py

@@ -1,20 +1,30 @@
 """REST API for dashboard widget instances and registry metadata."""
 
+import logging
+import time
 from typing import Any
 
 from fastapi import APIRouter, Depends, HTTPException, status
 
 from media_library_viewer_api.dependencies import get_settings_store
-from media_library_viewer_api.models.widgets import WidgetInstance, WidgetInstanceInput, WidgetTypeInfo
+from media_library_viewer_api.models.widgets import (
+    WidgetDataResponse,
+    WidgetInstance,
+    WidgetInstanceInput,
+)
 from media_library_viewer_api.services.settings_store import SettingsStore
 from media_library_viewer_api.widgets.registry import (
+    get_widget_info,
     list_source_types,
     list_widget_types,
     validate_config,
 )
+from media_library_viewer_api.widgets.sources import get_source_adapter
 
 router = APIRouter(prefix="/api/widgets", tags=["widgets"])
 
+logger = logging.getLogger(__name__)
+
 
 def _registry_for_type(widget_type: str) -> dict[str, Any]:
     from media_library_viewer_api.widgets.registry import WIDGET_REGISTRY
@@ -56,7 +66,7 @@ def list_sources() -> list[str]:
 
 
 @router.get("/types")
-def list_types() -> list[WidgetTypeInfo]:
+def list_types() -> list[dict[str, Any]]:
     """Return metadata for all registered widget types."""
     return [info.model_dump() for info in list_widget_types()]
 
@@ -111,3 +121,53 @@ def delete_instance(
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Widget not found")
     store.delete_widget(widget_id)
     return {"status": "deleted"}
+
+
+@router.get("/instances/{widget_id}/data")
+async def fetch_data(
+    widget_id: str,
+    store: SettingsStore = Depends(get_settings_store),
+) -> dict[str, Any]:
+    """Fetch widget data through the registered source adapter."""
+    widget = store.get_widget(widget_id)
+    if not widget:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Widget not found")
+
+    widget_type = widget["widget_type"]
+    info = get_widget_info(widget_type)
+    if info is None:
+        return WidgetDataResponse(
+            widget_id=widget_id,
+            widget_type=widget_type,
+            data=None,
+            error=f"Unknown widget type: {widget_type}",
+            fetched_at=int(time.time()),
+        ).model_dump()
+
+    adapter = get_source_adapter(info.source_type)
+    if adapter is None:
+        # Defensive: registry should prevent this, but return a safe error.
+        return WidgetDataResponse(
+            widget_id=widget_id,
+            widget_type=widget_type,
+            data=None,
+            error=f"No adapter registered for source type: {info.source_type}",
+            fetched_at=int(time.time()),
+        ).model_dump()
+
+    try:
+        data = await adapter.fetch(widget["config"])
+    except Exception as exc:
+        logger.exception("Unhandled adapter exception widget_id=%s", widget_id)
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail="Widget data fetch failed",
+        ) from exc
+
+    return WidgetDataResponse(
+        widget_id=widget_id,
+        widget_type=widget_type,
+        data=data if "error" not in data else None,
+        error=data.get("error"),
+        fetched_at=int(time.time()),
+    ).model_dump()