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-5-frontend-templates.md

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+# Phase 5: Frontend — Audio Processing Templates
+
+**Status:** ⬜ Not Started
+**Parent plan:** [PLAN.md](./PLAN.md)
+**Domain:** frontend
+
+## Objective
+Build the frontend UI for managing Audio Processing Templates — list, create, edit, delete, with a filter editor and real-time preview.
+
+## Tasks
+
+- [ ] Task 1: Create TypeScript module `static/js/features/audio-processing-templates.ts`
+  - Fetch/cache audio processing templates via DataCache
+  - CRUD operations using fetchWithAuth
+  - CardSection for template list with reconciliation
+- [ ] Task 2: Create template editor modal
+  - Name, description, tags fields
+  - Ordered filter list with add/remove/reorder controls
+  - Per-filter option controls (sliders, selects, toggles) driven by option schemas from `GET /api/v1/audio-filters`
+  - Template composition support: `audio_filter_template` shows EntitySelect for sub-template
+  - Dirty check via snapshotValues()
+- [ ] Task 3: Add Audio Processing Templates section to the Streams tab
+  - New sub-tab or section alongside existing Audio Sources
+  - CardSection rendering with template name, filter count, description
+  - Create/Edit/Delete actions per card
+- [ ] Task 4: Real-time audio preview
+  - "Test" button on template editor that opens a WebSocket connection
+  - Shows spectrum visualization (reuse existing audio test pattern)
+  - Applies template's filters to a selected source in real-time
+  - Source picker (EntitySelect) for choosing input audio source
+- [ ] Task 5: Add i18n keys for all new UI strings (en.json, ru.json, zh.json)
+  - Template section labels, filter names, option labels, buttons, errors
+- [ ] Task 6: Register module in `app.js` / global exports if needed for inline onclick handlers
+- [ ] Task 7: Fetch and cache audio filter registry data (for building filter option UIs)
+
+## Files to Modify/Create
+- `static/js/features/audio-processing-templates.ts` — **create** — main module
+- `static/js/features/audio-processing-template-modal.ts` — **create** — editor modal
+- `static/css/dashboard.css` — **modify** — styles for template editor
+- `static/js/app.js` — **modify** — register module, add window exports
+- `templates/dashboard.html` (or relevant Jinja template) — **modify** — add section
+- `static/js/core/i18n/en.json` — **modify** — new keys
+- `static/js/core/i18n/ru.json` — **modify** — new keys
+- `static/js/core/i18n/zh.json` — **modify** — new keys
+
+## Acceptance Criteria
+- Audio Processing Templates section visible in Streams tab
+- Templates can be created, edited, deleted
+- Filter editor shows all 11 available filters with correct option controls
+- Template composition (audio_filter_template) works via EntitySelect
+- Real-time preview shows filtered audio data
+- All strings are internationalized
+
+## Notes
+- Follow existing patterns from postprocessing template UI
+- Use IconSelect for filter type selection (if icon set supports it) or a custom filter picker
+- NEVER use plain HTML `<select>` — use project custom selectors (CRITICAL project rule)
+- NEVER use emoji — use SVG icons from `core/icons.ts`
+- Use fetchWithAuth for ALL API calls (project rule)
+- Call cache.invalidate() before load functions in save/delete handlers
+
+## Review Checklist
+- [ ] All tasks completed
+- [ ] Code follows project conventions
+- [ ] No unintended side effects
+- [ ] Build passes
+- [ ] Tests pass (new + existing)
+
+## Handoff to Next Phase
+<!-- Filled in by the implementation agent after completing this phase. -->