Add audio-reactive color strip sources, improve delete error messages

Add new "audio" color strip source type with three visualization modes
(spectrum analyzer, beat pulse, VU meter) supporting WASAPI loopback and
microphone input via PyAudioWPatch. Includes shared audio capture with
ref counting, real-time FFT spectrum analysis, and beat detection.

Improve all referential integrity 409 error messages across delete
endpoints to include specific names of referencing entities instead of
generic "one or more" messages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -142,10 +142,12 @@ async def delete_pp_template(
     """Delete a postprocessing template."""
     try:
         # Check if any picture source references this template
-        if store.is_referenced_by(template_id, stream_store):
+        source_names = store.get_sources_referencing(template_id, stream_store)
+        if source_names:
+            names = ", ".join(source_names)
             raise HTTPException(
                 status_code=409,
-                detail="Cannot delete postprocessing template: it is referenced by one or more picture sources. "
+                detail=f"Cannot delete postprocessing template: it is referenced by picture source(s): {names}. "
                        "Please reassign those streams before deleting.",
             )
         store.delete_template(template_id)