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/api/schemas/audio_templates.py

@@ -0,0 +1,59 @@
+"""Audio capture template and engine schemas."""
+
+from datetime import datetime
+from typing import Dict, List, Optional
+
+from pydantic import BaseModel, Field
+
+
+class AudioTemplateCreate(BaseModel):
+    """Request to create an audio capture template."""
+
+    name: str = Field(description="Template name", min_length=1, max_length=100)
+    engine_type: str = Field(description="Audio engine type (e.g., 'wasapi', 'sounddevice')", min_length=1)
+    engine_config: Dict = Field(default_factory=dict, description="Engine-specific configuration")
+    description: Optional[str] = Field(None, description="Template description", max_length=500)
+
+
+class AudioTemplateUpdate(BaseModel):
+    """Request to update an audio template."""
+
+    name: Optional[str] = Field(None, description="Template name", min_length=1, max_length=100)
+    engine_type: Optional[str] = Field(None, description="Audio engine type")
+    engine_config: Optional[Dict] = Field(None, description="Engine-specific configuration")
+    description: Optional[str] = Field(None, description="Template description", max_length=500)
+
+
+class AudioTemplateResponse(BaseModel):
+    """Audio template information response."""
+
+    id: str = Field(description="Template ID")
+    name: str = Field(description="Template name")
+    engine_type: str = Field(description="Engine type identifier")
+    engine_config: Dict = Field(description="Engine-specific configuration")
+    created_at: datetime = Field(description="Creation timestamp")
+    updated_at: datetime = Field(description="Last update timestamp")
+    description: Optional[str] = Field(None, description="Template description")
+
+
+class AudioTemplateListResponse(BaseModel):
+    """List of audio templates response."""
+
+    templates: List[AudioTemplateResponse] = Field(description="List of audio templates")
+    count: int = Field(description="Number of templates")
+
+
+class AudioEngineInfo(BaseModel):
+    """Audio capture engine information."""
+
+    type: str = Field(description="Engine type identifier (e.g., 'wasapi', 'sounddevice')")
+    name: str = Field(description="Human-readable engine name")
+    default_config: Dict = Field(description="Default configuration for this engine")
+    available: bool = Field(description="Whether engine is available on this system")
+
+
+class AudioEngineListResponse(BaseModel):
+    """List of audio engines response."""
+
+    engines: List[AudioEngineInfo] = Field(description="Available audio engines")
+    count: int = Field(description="Number of engines")