feat(processed-audio-sources): phase 1 - audio filter framework

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

plans/processed-audio-sources/PLAN.md

@@ -0,0 +1,55 @@
+# Feature: Processed Audio Sources
+
+**Branch:** `feature/processed-audio-sources`
+**Base branch:** `master`
+**Created:** 2026-03-31
+**Status:** 🟡 In Progress
+**Strategy:** Big Bang
+**Mode:** Automated
+**Execution:** Orchestrator
+
+## Summary
+
+Replace hardcoded `MonoAudioSource` and `BandExtractAudioSource` types with a composable
+**ProcessedAudioSource + AudioProcessingTemplate + AudioFilter** system — mirroring the
+existing processed picture source pattern. Rename `MultichannelAudioSource` to
+`CaptureAudioSource`. Adds 11 audio filters: channel extract, band extract, peak hold,
+gain, noise gate, envelope follower, spectral smoothing, compressor, inverter, beat gate,
+and delay.
+
+Clean-slate approach: no data migration for old source types.
+
+## Build & Test Commands
+- **Build (Python):** `cd server && ruff check src/ tests/ --fix`
+- **Build (TypeScript):** `cd server && npx tsc --noEmit && npm run build`
+- **Test:** `cd server && py -3.13 -m pytest tests/ --no-cov -q`
+
+## Phases
+
+- [ ] Phase 1: Audio Filter Framework [domain: backend] → [subplan](./phase-1-audio-filter-framework.md)
+- [ ] Phase 2: Audio Filters [domain: backend] → [subplan](./phase-2-audio-filters.md)
+- [ ] Phase 3: Processed Audio Source Model [domain: backend] → [subplan](./phase-3-processed-audio-source-model.md)
+- [ ] Phase 4: Runtime Integration [domain: backend] → [subplan](./phase-4-runtime-integration.md)
+- [ ] Phase 5: Frontend — Audio Processing Templates [domain: frontend] → [subplan](./phase-5-frontend-templates.md)
+- [ ] Phase 6: Frontend — Source Types [domain: frontend] → [subplan](./phase-6-frontend-source-types.md)
+- [ ] Phase 7: Testing & Polish [domain: backend] → [subplan](./phase-7-testing-polish.md)
+- [ ] Phase 8: Frontend Design Consistency Review [domain: frontend] → [subplan](./phase-8-frontend-design-review.md)
+
+## Phase Progress Log
+
+| Phase | Domain | Status | Review | Build | Committed |
+|-------|--------|--------|--------|-------|-----------|
+| Phase 1: Audio Filter Framework | backend | 🔨 In Progress | ⬜ | ⬜ | ⬜ |
+| Phase 2: Audio Filters | backend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 3: Processed Audio Source Model | backend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 4: Runtime Integration | backend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 5: Frontend — Audio Processing Templates | frontend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 6: Frontend — Source Types | frontend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 7: Testing & Polish | backend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+| Phase 8: Frontend Design Review | frontend | ⬜ Not Started | ⬜ | ⬜ | ⬜ |
+
+## Final Review
+- [ ] Comprehensive code review
+- [ ] Full build passes
+- [ ] Full test suite passes
+- [ ] Merged to `master`