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.

backend/src/media_library_viewer_api/main.py

@@ -23,6 +23,7 @@ from media_library_viewer_api.observability import (
 )
 from media_library_viewer_api.routers import backups as backups_router
 from media_library_viewer_api.routers import dashboard, files, jobs, media, monitoring, tasks, users
+from media_library_viewer_api.routers import widgets as widgets_router
 from media_library_viewer_api.routers.settings import router as settings_router
 
 from .services.backup_poller import get_backup_poller
@@ -45,6 +46,10 @@ async def lifespan(app: FastAPI):
         write_prometheus_targets(get_settings_store())
     except Exception:
         logger.exception("Failed to write Prometheus file-SD targets during startup")
+    try:
+        get_settings_store().ensure_defaults()
+    except Exception:
+        logger.exception("Failed to seed default settings during startup")
     mail_queue = get_mail_queue()
     backup_poller = get_backup_poller()
     mail_queue.start()
@@ -137,6 +142,7 @@ app.include_router(users.router)
 app.include_router(tasks.router)
 app.include_router(settings_router)
 app.include_router(backups_router.router)
+app.include_router(widgets_router.router)
 
 
 @app.get("/api/health")