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/CLAUDE.md

@@ -7,7 +7,7 @@
 - `src/wled_controller/api/schemas/` — Pydantic request/response models (one file per entity)
 - `src/wled_controller/core/` — Core business logic (capture, devices, audio, processing, automations)
 - `src/wled_controller/storage/` — Data models (dataclasses) and JSON persistence stores
-- `src/wled_controller/utils/` — Utility functions (logging, monitor detection)
+- `src/wled_controller/utils/` — Utility functions (logging, monitor detection, SSRF validation, sound playback)
 - `src/wled_controller/static/` — Frontend files (TypeScript, CSS, locales)
 - `src/wled_controller/templates/` — Jinja2 HTML templates
 - `config/` — Configuration files (YAML)