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>

server/src/wled_controller/templates/modals/audio-template.html

@@ -0,0 +1,45 @@
+<!-- Audio Template Modal -->
+<div id="audio-template-modal" class="modal" role="dialog" aria-modal="true" aria-labelledby="audio-template-modal-title">
+    <div class="modal-content">
+        <div class="modal-header">
+            <h2 id="audio-template-modal-title" data-i18n="audio_template.add">Add Audio Template</h2>
+            <button class="modal-close-btn" onclick="closeAudioTemplateModal()" title="Close" data-i18n-aria-label="aria.close">&#x2715;</button>
+        </div>
+        <div class="modal-body">
+            <input type="hidden" id="audio-template-id">
+            <form id="audio-template-form">
+                <div class="form-group">
+                    <label for="audio-template-name" data-i18n="audio_template.name">Template Name:</label>
+                    <input type="text" id="audio-template-name" data-i18n-placeholder="audio_template.name.placeholder" placeholder="My Audio Template" required>
+                </div>
+
+                <div class="form-group">
+                    <label for="audio-template-description" data-i18n="audio_template.description.label">Description (optional):</label>
+                    <input type="text" id="audio-template-description" data-i18n-placeholder="audio_template.description.placeholder" placeholder="Describe this template..." maxlength="500">
+                </div>
+
+                <div class="form-group">
+                    <div class="label-row">
+                        <label for="audio-template-engine" data-i18n="audio_template.engine">Audio Engine:</label>
+                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?">?</button>
+                    </div>
+                    <small class="input-hint" style="display:none" data-i18n="audio_template.engine.hint">Select the audio capture backend to use</small>
+                    <select id="audio-template-engine" onchange="onAudioEngineChange()" required>
+                    </select>
+                    <small id="audio-engine-availability-hint" class="form-hint" style="display: none;"></small>
+                </div>
+
+                <div id="audio-engine-config-section" style="display: none;">
+                    <h3 data-i18n="audio_template.config">Configuration</h3>
+                    <div id="audio-engine-config-fields"></div>
+                </div>
+
+                <div id="audio-template-error" class="error-message" style="display: none;"></div>
+            </form>
+        </div>
+        <div class="modal-footer">
+            <button class="btn btn-icon btn-secondary" onclick="closeAudioTemplateModal()" title="Cancel" data-i18n-aria-label="aria.cancel">&#x2715;</button>
+            <button class="btn btn-icon btn-primary" onclick="saveAudioTemplate()" title="Save" data-i18n-aria-label="aria.save">&#x2713;</button>
+        </div>
+    </div>
+</div>