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/phase-8-frontend-design-review.md

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+# Phase 8: Frontend Design Consistency Review
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** frontend
+
+## Objective
+Review all new frontend UI created in Phases 5-6 for visual consistency, design quality, and UX polish using the frontend-design skill agent. Fix any issues found.
+
+## Tasks
+
+- [ ] Task 1: Review Audio Processing Templates section for design consistency
+  - Card layout, spacing, typography alignment with existing sections
+  - Filter editor modal — controls alignment, visual hierarchy, grouping
+  - Responsive behavior at different viewport widths
+- [ ] Task 2: Review Processed Audio Source cards for design consistency
+  - Card style matches existing source cards (capture, picture, value sources)
+  - EntitySelect pickers are visually consistent
+  - Type badges/icons are clear and distinguishable
+- [ ] Task 3: Review Capture Audio Source card (relabeled)
+  - Label/icon updates look correct
+  - No visual regressions from the rename
+- [ ] Task 4: Review source type picker/creation flow
+  - Type selector is clear and accessible
+  - Transition between types is smooth
+  - Empty states handled properly
+- [ ] Task 5: Review real-time audio preview UI
+  - Spectrum visualization looks polished
+  - Source picker and controls are well-placed
+  - Loading/error states
+- [ ] Task 6: Fix all design issues found in Tasks 1-5
+  - CSS adjustments for spacing, alignment, typography
+  - Icon/color consistency
+  - Dark mode compatibility (if applicable)
+  - Hover/focus/active states on interactive elements
+- [ ] Task 7: Cross-browser spot-check (if applicable)
+
+## Files to Modify/Create
+- `static/css/dashboard.css` — **modify** — design fixes
+- `static/js/features/audio-processing-templates.ts` — **modify** — UX fixes
+- `static/js/features/audio-processing-template-modal.ts` — **modify** — UX fixes
+- `static/js/features/audio-sources.ts` — **modify** — UX fixes
+- Any template/HTML files — **modify** — structural fixes
+
+## Acceptance Criteria
+- All new UI sections are visually consistent with existing sections
+- No orphaned styles or visual regressions
+- Filter editor is intuitive and well-organized
+- Cards, modals, and controls follow existing design language
+- Interactive elements have proper hover/focus/active states
+
+## Notes
+- This phase uses the frontend-design skill agent for review
+- Focus on consistency with existing UI, not a complete redesign
+- The project uses vanilla CSS (no framework) — fixes must use the existing stylesheet approach
+
+## Review Checklist
+- [ ] All tasks completed
+- [ ] Code follows project conventions
+- [ ] No unintended side effects
+- [ ] Build passes
+- [ ] Tests pass (new + existing)
+
+## Handoff to Next Phase
+<!-- N/A — final phase -->