feat(widgets): add backend CRUD, registry, and default seeding
Introduce a closed, compile-time widget registry and backend CRUD for
dashboard widget instances.
- Add dashboard_widgets SQLite table in SettingsStore with CRUD helpers and
default seeding (Jellyfin + Backups) on first install.
- Add Pydantic models with credential-key and secret-value rejection.
- Add widgets router: /api/widgets/sources, /types, /instances CRUD.
- Call ensure_defaults() in app lifespan so fresh installs seed defaults.
- Add backend tests covering registry, CRUD, validation, and seeding.
- Include SDD artifacts: exploration, proposal, spec, design, tasks.
Developer
200d319fb0