feat(processed-audio-sources): phase 1 - audio filter framework

Add the foundation for audio processing filters, mirroring the existing
picture filter/postprocessing template system:
- AudioFilter base class, AudioFilterRegistry, AudioFilterOptionDef
- AudioProcessingTemplate dataclass + SQLite-backed store
- audio_filter_template meta-filter with recursive resolution
- Full REST API: CRUD templates + filter registry discovery
- Dependency injection wired in dependencies.py and main.py

server/src/wled_controller/storage/audio_processing_template.py

@@ -0,0 +1,55 @@
+"""Audio processing template data model."""
+
+from dataclasses import dataclass, field
+from datetime import datetime, timezone
+from typing import List, Optional
+
+from wled_controller.core.filters.filter_instance import FilterInstance
+
+
+@dataclass
+class AudioProcessingTemplate:
+    """Audio processing settings template containing an ordered list of audio filters."""
+
+    id: str
+    name: str
+    filters: List[FilterInstance]
+    created_at: datetime
+    updated_at: datetime
+    description: Optional[str] = None
+    tags: List[str] = field(default_factory=list)
+
+    def to_dict(self) -> dict:
+        """Convert template to dictionary."""
+        return {
+            "id": self.id,
+            "name": self.name,
+            "filters": [f.to_dict() for f in self.filters],
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+            "description": self.description,
+            "tags": self.tags,
+        }
+
+    @classmethod
+    def from_dict(cls, data: dict) -> "AudioProcessingTemplate":
+        """Create template from dictionary."""
+        filters = [FilterInstance.from_dict(f) for f in data.get("filters", [])]
+
+        return cls(
+            id=data["id"],
+            name=data["name"],
+            filters=filters,
+            created_at=(
+                datetime.fromisoformat(data["created_at"])
+                if isinstance(data.get("created_at"), str)
+                else data.get("created_at", datetime.now(timezone.utc))
+            ),
+            updated_at=(
+                datetime.fromisoformat(data["updated_at"])
+                if isinstance(data.get("updated_at"), str)
+                else data.get("updated_at", datetime.now(timezone.utc))
+            ),
+            description=data.get("description"),
+            tags=data.get("tags", []),
+        )