Add all audio filters that transform AudioAnalysis data:
- Channel Extract, Band Extract (migration from old source types)
- Peak Hold, Gain, Noise Gate, Envelope Follower
- Spectral Smoothing, Compressor, Inverter, Beat Gate, Delay
All registered via AudioFilterRegistry with option schemas.
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