Add audio capture engine template system with multi-backend support

Introduces an engine+template abstraction for audio capture, mirroring the
existing screen capture engine pattern. This enables multiple audio backends
(WASAPI for Windows, sounddevice for cross-platform) with per-source
engine configuration via reusable templates.

Backend:
- AudioCaptureEngine ABC with WasapiEngine and SounddeviceEngine implementations
- AudioEngineRegistry for engine discovery and factory creation
- AudioAnalyzer class decouples FFT/RMS/beat analysis from engine-specific capture
- ManagedAudioStream wraps engine stream + analyzer in background thread
- AudioCaptureTemplate model and AudioTemplateStore with JSON CRUD
- AudioCaptureManager keyed by (engine_type, device_index, is_loopback)
- Auto-migration: default template created on startup, assigned to existing sources
- Full REST API: CRUD for audio templates + engine listing with availability flags
- audio_template_id added to MultichannelAudioSource model and API schemas

Frontend:
- Audio template cards in Streams > Audio tab with engine badge and config details
- Audio template editor modal with engine selector and dynamic config fields
- Audio template dropdown in multichannel audio source editor
- Template name crosslink badge on multichannel audio source cards
- Confirm modal z-index fix (always stacks above editor modals)
- i18n keys for EN and RU

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/templates/modals/audio-source-editor.html

@@ -36,6 +36,17 @@
 
                 <!-- Multichannel fields -->
                 <div id="audio-source-multichannel-section">
+                    <div class="form-group">
+                        <div class="label-row">
+                            <label for="audio-source-audio-template" data-i18n="audio_source.audio_template">Audio Template:</label>
+                            <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>
+                        </div>
+                        <small class="input-hint" style="display:none" data-i18n="audio_source.audio_template.hint">Audio capture template that defines which engine and settings to use for this device</small>
+                        <select id="audio-source-audio-template">
+                            <!-- populated dynamically -->
+                        </select>
+                    </div>
+
                     <div class="form-group">
                         <div class="label-row">
                             <label for="audio-source-device" data-i18n="audio_source.device">Audio Device:</label>