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/static/js/app.ts

@@ -137,7 +137,7 @@ import {
     previewCSSFromEditor,
     copyEndpointUrl,
     onNotificationFilterModeChange,
-    notificationAddAppColor, notificationRemoveAppColor,
+    notificationAddAppOverride, notificationRemoveAppOverride,
     testNotification,
     showNotificationHistory, closeNotificationHistory, refreshNotificationHistory,
     testColorStrip, testCSPT, closeTestCssSourceModal, applyCssTestSettings, fireCssTestNotification, fireCssTestNotificationLayer,
@@ -464,7 +464,7 @@ Object.assign(window, {
     previewCSSFromEditor,
     copyEndpointUrl,
     onNotificationFilterModeChange,
-    notificationAddAppColor, notificationRemoveAppColor,
+    notificationAddAppOverride, notificationRemoveAppOverride,
     testNotification,
     showNotificationHistory, closeNotificationHistory, refreshNotificationHistory,
     testColorStrip, testCSPT, closeTestCssSourceModal, applyCssTestSettings, fireCssTestNotification, fireCssTestNotificationLayer,