feat(processed-audio-sources): phase 2 - implement 11 audio filters

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.

plans/processed-audio-sources/CONTEXT.md

@@ -9,13 +9,20 @@
 - **Test:** `cd server && py -3.13 -m pytest tests/ --no-cov -q`
 
 ## Current State
-Phase 1 (Audio Filter Framework) implemented. Core framework is in place:
+Phase 1 (Audio Filter Framework) and Phase 2 (Audio Filters) implemented.
+
+Phase 1 framework:
 - `AudioFilter` base class, `AudioFilterRegistry`, `AudioFilterOptionDef` in `core/audio/filters/`
 - `AudioProcessingTemplate` dataclass + `AudioProcessingTemplateStore` (SQLite-backed) in `storage/`
 - `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`
 
+Phase 2 filters (12 total registered, 11 real + 1 meta):
+- Stateless: `channel_extract`, `band_extract`, `gain`, `inverter`
+- Stateful: `peak_hold`, `noise_gate`, `envelope_follower`, `spectral_smoothing`, `compressor`, `beat_gate`, `delay`
+- All produce new `AudioAnalysis` via `dataclasses.replace()` (immutability preserved)
+
 ## Key Architecture Reference
 
 ### Existing Pattern to Mirror: Processed Picture Sources
@@ -83,7 +90,7 @@ _(none yet)_
 | Phase | Agent Used | Test Writer | Parallel | Notes |
 |-------|-----------|-------------|----------|-------|
 | Phase 1 | impl-agent | — | No | Tasks 7+8 skipped (SQLite migration made them obsolete) |
-| Phase 2 | — | — | — | — |
+| Phase 2 | impl-agent | — | No | All 11 filters implemented, no deviations |
 | Phase 3 | — | — | — | — |
 | Phase 4 | — | — | — | — |
 | Phase 5 | — | — | — | — |
@@ -98,6 +105,6 @@ _(none yet)_
 
 ## Implementation Notes
 - Clean-slate approach: no migration of existing MonoAudioSource/BandExtractAudioSource data
-- 5 of 11 filters are stateful (peak hold, envelope follower, spectral smoothing, compressor, delay) — need per-stream instance lifecycle
+- 7 of 11 filters are stateful (peak hold, noise gate, envelope follower, spectral smoothing, compressor, beat gate, delay) — need per-stream instance lifecycle
 - Audio filters operate on AudioAnalysis snapshots, not raw audio samples
 - Big Bang strategy: intermediate phases may break the build; only Phase 7 enforces build/tests