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

TODO.md

@@ -1,37 +0,0 @@
-# Build Size Reduction
-
-## Phase 1: Quick Wins (build scripts)
-
-- [x] Strip unused NumPy submodules (polynomial, linalg, ma, lib, distutils)
-- [x] Strip debug symbols from .pyd/.dll/.so files
-- [x] Remove zeroconf service database
-- [x] Remove .py source from site-packages after compiling to .pyc
-- [x] Strip unused PIL image plugins (keep JPEG/PNG/ICO/BMP for tray)
-
-## Phase 2: Replace Pillow with cv2
-
-- [x] Create `utils/image_codec.py` with cv2-based image helpers
-- [x] Replace PIL in `_preview_helpers.py`
-- [x] Replace PIL in `picture_sources.py`
-- [x] Replace PIL in `color_strip_sources.py`
-- [x] Replace PIL in `templates.py`
-- [x] Replace PIL in `postprocessing.py`
-- [x] Replace PIL in `output_targets_keycolors.py`
-- [x] Replace PIL in `kc_target_processor.py`
-- [x] Replace PIL in `pixelate.py` filter
-- [x] Replace PIL in `downscaler.py` filter
-- [x] Replace PIL in `scrcpy_engine.py`
-- [x] Replace PIL in `live_stream_manager.py`
-- [x] Move Pillow from core deps to [tray] optional in pyproject.toml
-- [x] Make PIL import conditional in `tray.py`
-- [x] Move opencv-python-headless to core dependencies
-
-## Phase 4: OpenCV stripping (build scripts)
-
-- [x] Strip ffmpeg DLL, Haar cascades, dev files (already existed)
-- [x] Strip typing stubs (already existed)
-
-## Verification
-
-- [x] Lint: `ruff check src/ tests/ --fix`
-- [x] Tests: 341 passed