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/config.py

@@ -57,6 +57,8 @@ class Settings(BaseSettings):
     prometheus_file_sd_dir: str = "/app/backend/.cache/prometheus-file-sd"
     alertmanager_url: str = "http://alertmanager:9093"
     alertmanager_webhook_url: str = ""  # Optional receiver for alertmanager webhook notifications
+    grafana_url: str = "http://grafana:3000"
+    prometheus_url: str = "http://prometheus:9090"
 
     # Remote paths
     remote_media_root: str = ""