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/README.md

@@ -8,7 +8,7 @@ The server component provides:
 - 🎯 **Real-time Screen Capture** - Multi-monitor support with configurable FPS
 - 🎨 **Advanced Processing** - Border pixel extraction with color correction
 - 🔧 **Flexible Calibration** - Map screen edges to any LED layout
-- 🌐 **REST API** - Complete control via 17 REST endpoints
+- 🌐 **REST API** - Complete control via 25+ REST endpoints
 - 💾 **Persistent Storage** - JSON-based device and configuration management
 - 📊 **Metrics & Monitoring** - Real-time FPS, status, and performance data