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/storage/audio_source.py

@@ -33,6 +33,7 @@ class AudioSource:
             # Subclass fields default to None for forward compat
             "device_index": None,
             "is_loopback": None,
+            "audio_template_id": None,
             "audio_source_id": None,
             "channel": None,
         }
@@ -74,6 +75,7 @@ class AudioSource:
             created_at=created_at, updated_at=updated_at, description=description,
             device_index=int(data.get("device_index", -1)),
             is_loopback=bool(data.get("is_loopback", True)),
+            audio_template_id=data.get("audio_template_id"),
         )
 
 
@@ -87,11 +89,13 @@ class MultichannelAudioSource(AudioSource):
 
     device_index: int = -1      # -1 = default device
     is_loopback: bool = True    # True = WASAPI loopback (system audio)
+    audio_template_id: Optional[str] = None  # references AudioCaptureTemplate
 
     def to_dict(self) -> dict:
         d = super().to_dict()
         d["device_index"] = self.device_index
         d["is_loopback"] = self.is_loopback
+        d["audio_template_id"] = self.audio_template_id
         return d