Rewrite audio source editor modal for capture/processed types only.
Remove old multichannel/mono/band_extract HTML sections and i18n keys.
Clean up legacy DOM section null-checks in audio-sources.ts.
Add the foundation for audio processing filters, mirroring the existing
picture filter/postprocessing template system:
- AudioFilter base class, AudioFilterRegistry, AudioFilterOptionDef
- AudioProcessingTemplate dataclass + SQLite-backed store
- audio_filter_template meta-filter with recursive resolution
- Full REST API: CRUD templates + filter registry discovery
- Dependency injection wired in dependencies.py and main.py
alexei.dolgolyov