fix: add reference check before deleting audio processing template

Prevent deleting templates that are still referenced by
ProcessedAudioSource entities. Returns 400 with source names.

server/src/wled_controller/api/routes/audio_processing_templates.py

@@ -6,6 +6,7 @@ from wled_controller.api.auth import AuthRequired
 from wled_controller.api.dependencies import (
     fire_entity_event,
     get_audio_processing_template_store,
+    get_audio_source_store,
     get_processor_manager,
 )
 from wled_controller.api.schemas.audio_processing import (
@@ -158,6 +159,12 @@ async def delete_audio_processing_template(
 ):
     """Delete an audio processing template."""
     try:
+        # Check for references from audio sources
+        audio_source_store = get_audio_source_store()
+        refs = audio_source_store.get_sources_referencing_template(template_id)
+        if refs:
+            names = ", ".join(r.name for r in refs)
+            raise ValueError(f"Template is in use by audio source(s): {names}")
         store.delete_template(template_id)
         fire_entity_event("audio_processing_template", "deleted", template_id)
         # Hot-update: rebuild filter pipelines for running streams that used this template