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.

docker-compose.yml

@@ -28,6 +28,8 @@ services:
       PROMETHEUS_FILE_SD_DIR: ${PROMETHEUS_FILE_SD_DIR:-/app/backend/.cache/prometheus-file-sd}
       ALERTMANAGER_URL: ${ALERTMANAGER_URL:-http://alertmanager:9093}
       ALERTMANAGER_WEBHOOK_URL: ${ALERTMANAGER_WEBHOOK_URL:-}
+      GRAFANA_URL: ${GRAFANA_URL:-http://grafana:3000}
+      PROMETHEUS_URL: ${PROMETHEUS_URL:-http://prometheus:9090}
     volumes:
       - ${BACKEND_CACHE_DIR:-./backend-cache}:/app/backend/.cache
     restart: unless-stopped