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.

.env.example

@@ -23,6 +23,8 @@ PROMETHEUS_ENABLED=true
 PROMETHEUS_FILE_SD_DIR=/app/backend/.cache/prometheus-file-sd
 ALERTMANAGER_URL=http://alertmanager:9093
 ALERTMANAGER_WEBHOOK_URL=
+GRAFANA_URL=http://grafana:3000
+PROMETHEUS_URL=http://prometheus:9090
 BACKEND_CACHE_DIR=./backend-cache
 
 # Auth