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_sources.py

@@ -14,6 +14,7 @@ class AudioSourceCreate(BaseModel):
     # multichannel fields
     device_index: Optional[int] = Field(None, description="Audio device index (-1 = default)")
     is_loopback: Optional[bool] = Field(None, description="True for system audio (WASAPI loopback)")
+    audio_template_id: Optional[str] = Field(None, description="Audio capture template ID")
     # mono fields
     audio_source_id: Optional[str] = Field(None, description="Parent multichannel audio source ID")
     channel: Optional[str] = Field(None, description="Channel: mono|left|right")
@@ -26,6 +27,7 @@ class AudioSourceUpdate(BaseModel):
     name: Optional[str] = Field(None, description="Source name", min_length=1, max_length=100)
     device_index: Optional[int] = Field(None, description="Audio device index (-1 = default)")
     is_loopback: Optional[bool] = Field(None, description="True for system audio (WASAPI loopback)")
+    audio_template_id: Optional[str] = Field(None, description="Audio capture template ID")
     audio_source_id: Optional[str] = Field(None, description="Parent multichannel audio source ID")
     channel: Optional[str] = Field(None, description="Channel: mono|left|right")
     description: Optional[str] = Field(None, description="Optional description", max_length=500)
@@ -39,6 +41,7 @@ class AudioSourceResponse(BaseModel):
     source_type: str = Field(description="Source type: multichannel or mono")
     device_index: Optional[int] = Field(None, description="Audio device index")
     is_loopback: Optional[bool] = Field(None, description="WASAPI loopback mode")
+    audio_template_id: Optional[str] = Field(None, description="Audio capture template ID")
     audio_source_id: Optional[str] = Field(None, description="Parent multichannel source ID")
     channel: Optional[str] = Field(None, description="Channel: mono|left|right")
     description: Optional[str] = Field(None, description="Description")