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.
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