feat: add band_extract audio source type for frequency band filtering

New audio source type that filters a parent source to a specific frequency
band (bass 20-250Hz, mid 250-4kHz, treble 4k-20kHz, or custom range).
Supports chaining with frequency range intersection and cycle detection.
Band filtering applied in both CSS audio streams and test WebSocket.

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

@@ -42,6 +42,9 @@ def _to_response(source: AudioSource) -> AudioSourceResponse:
         audio_template_id=getattr(source, "audio_template_id", None),
         audio_source_id=getattr(source, "audio_source_id", None),
         channel=getattr(source, "channel", None),
+        band=getattr(source, "band", None),
+        freq_low=getattr(source, "freq_low", None),
+        freq_high=getattr(source, "freq_high", None),
         description=source.description,
         tags=source.tags,
         created_at=source.created_at,
@@ -52,7 +55,7 @@ def _to_response(source: AudioSource) -> AudioSourceResponse:
 @router.get("/api/v1/audio-sources", response_model=AudioSourceListResponse, tags=["Audio Sources"])
 async def list_audio_sources(
     _auth: AuthRequired,
-    source_type: Optional[str] = Query(None, description="Filter by source_type: multichannel or mono"),
+    source_type: Optional[str] = Query(None, description="Filter by source_type: multichannel, mono, or band_extract"),
     store: AudioSourceStore = Depends(get_audio_source_store),
 ):
     """List all audio sources, optionally filtered by type."""
@@ -83,6 +86,9 @@ async def create_audio_source(
             description=data.description,
             audio_template_id=data.audio_template_id,
             tags=data.tags,
+            band=data.band,
+            freq_low=data.freq_low,
+            freq_high=data.freq_high,
         )
         fire_entity_event("audio_source", "created", source.id)
         return _to_response(source)
@@ -126,6 +132,9 @@ async def update_audio_source(
             description=data.description,
             audio_template_id=data.audio_template_id,
             tags=data.tags,
+            band=data.band,
+            freq_low=data.freq_low,
+            freq_high=data.freq_high,
         )
         fire_entity_event("audio_source", "updated", source_id)
         return _to_response(source)
@@ -182,17 +191,28 @@ async def test_audio_source_ws(
         await websocket.close(code=4001, reason="Unauthorized")
         return
 
-    # Resolve source → device info
+    # Resolve source → device info + optional band filter
     store = get_audio_source_store()
     template_store = get_audio_template_store()
     manager = get_processor_manager()
 
     try:
-        device_index, is_loopback, channel, audio_template_id = store.resolve_audio_source(source_id)
+        resolved = store.resolve_audio_source(source_id)
     except ValueError as e:
         await websocket.close(code=4004, reason=str(e))
         return
 
+    device_index = resolved.device_index
+    is_loopback = resolved.is_loopback
+    channel = resolved.channel
+    audio_template_id = resolved.audio_template_id
+
+    # Precompute band mask if this is a band_extract source
+    band_mask = None
+    if resolved.freq_low is not None and resolved.freq_high is not None:
+        from wled_controller.core.audio.band_filter import compute_band_mask
+        band_mask = compute_band_mask(resolved.freq_low, resolved.freq_high)
+
     # Resolve template → engine_type + config
     engine_type = None
     engine_config = None
@@ -233,6 +253,11 @@ async def test_audio_source_ws(
                     spectrum = analysis.spectrum
                     rms = analysis.rms
 
+                # Apply band filter if present
+                if band_mask is not None:
+                    from wled_controller.core.audio.band_filter import apply_band_filter
+                    spectrum, rms = apply_band_filter(spectrum, rms, band_mask)
+
                 await websocket.send_json({
                     "spectrum": spectrum.tolist(),
                     "rms": round(rms, 4),