feat: asset-based image/video sources, notification sounds, UI improvements

- Replace URL-based image_source/url fields with image_asset_id/video_asset_id
  on StaticImagePictureSource and VideoCaptureSource (clean break, no migration)
- Resolve asset IDs to file paths at runtime via AssetStore.get_file_path()
- Add EntitySelect asset pickers for image/video in stream editor modal
- Add notification sound configuration (global sound + per-app overrides)
- Unify per-app color and sound overrides into single "Per-App Overrides" section
- Persist notification history between server restarts
- Add asset management system (upload, edit, delete, soft-delete)
- Replace emoji buttons with SVG icons throughout UI
- Various backend improvements: SQLite stores, auth, backup, MQTT, webhooks

server/src/wled_controller/api/__init__.py

@@ -26,6 +26,7 @@ from .routes.color_strip_processing import router as cspt_router
 from .routes.gradients import router as gradients_router
 from .routes.weather_sources import router as weather_sources_router
 from .routes.update import router as update_router
+from .routes.assets import router as assets_router
 
 router = APIRouter()
 router.include_router(system_router)
@@ -52,5 +53,6 @@ router.include_router(cspt_router)
 router.include_router(gradients_router)
 router.include_router(weather_sources_router)
 router.include_router(update_router)
+router.include_router(assets_router)
 
 __all__ = ["router"]