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_template.py

@@ -0,0 +1,47 @@
+"""Audio capture template data model."""
+
+from dataclasses import dataclass
+from datetime import datetime
+from typing import Any, Dict, Optional
+
+
+@dataclass
+class AudioCaptureTemplate:
+    """Represents an audio capture template configuration."""
+
+    id: str
+    name: str
+    engine_type: str
+    engine_config: Dict[str, Any]
+    created_at: datetime
+    updated_at: datetime
+    description: Optional[str] = None
+
+    def to_dict(self) -> dict:
+        """Convert template to dictionary."""
+        return {
+            "id": self.id,
+            "name": self.name,
+            "engine_type": self.engine_type,
+            "engine_config": self.engine_config,
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+            "description": self.description,
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> "AudioCaptureTemplate":
+        """Create template from dictionary."""
+        return cls(
+            id=data["id"],
+            name=data["name"],
+            engine_type=data["engine_type"],
+            engine_config=data.get("engine_config", {}),
+            created_at=datetime.fromisoformat(data["created_at"])
+            if isinstance(data.get("created_at"), str)
+            else data.get("created_at", datetime.utcnow()),
+            updated_at=datetime.fromisoformat(data["updated_at"])
+            if isinstance(data.get("updated_at"), str)
+            else data.get("updated_at", datetime.utcnow()),
+            description=data.get("description"),
+        )