fix: KC color strip test preview — use LiveStreamManager instead of raw engine

BetterCam (DXGI) only supports one capture session per display, so creating
a second engine for the KC test produced no frames when a target was already
running. Now uses LiveStreamManager.acquire() which ref-counts and reuses
the running capture.

Also removed double postprocessing — ProcessedLiveStream already applies
filters, so re-applying them in the KC test halved the resolution (960x400
→ 240x100).

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

@@ -485,7 +485,6 @@ async def test_key_colors_ws(
         calculate_dominant_color,
         calculate_median_color,
     )
-    from wled_controller.core.filters import FilterRegistry, ImagePool
     from wled_controller.storage.picture_source import ScreenCapturePictureSource
     from wled_controller.utils.image_codec import encode_jpeg_data_uri, resize_down
 
@@ -497,7 +496,6 @@ async def test_key_colors_ws(
     source_store = get_picture_source_store()
     manager = get_processor_manager()
     device_store = get_device_store()
-    pp_store = get_pp_template_store()
 
     try:
         source = store.get_source(source_id)
@@ -546,11 +544,19 @@ async def test_key_colors_ws(
     await websocket.accept()
     logger.info(f"KC CSS test WS connected for {source_id} (fps={fps})")
 
+    # Use LiveStreamManager — reuses the already-running capture engine when a
+    # target is active.  BetterCam (DXGI) only supports one session per display,
+    # so creating a second engine would produce no frames.
     live_stream_mgr = manager._live_stream_manager
-    live_stream = None
-
     try:
         live_stream = await asyncio.to_thread(live_stream_mgr.acquire, source.picture_source_id)
+    except Exception as e:
+        logger.error(f"KC test: LiveStream acquire failed: {e}")
+        await websocket.send_text(ws_json.dumps({"type": "error", "detail": str(e)}))
+        await websocket.close(code=4003, reason=str(e))
+        return
+
+    try:
         prev_frame_ref = None
 
         while True:
@@ -564,26 +570,14 @@ async def test_key_colors_ws(
                     await asyncio.sleep(frame_interval * 0.5)
                     continue
                 prev_frame_ref = capture
-
                 cur_image = capture.image
+
                 if not isinstance(cur_image, np.ndarray):
                     await asyncio.sleep(frame_interval)
                     continue
 
-                # Apply postprocessing
-                pp_ids = chain.get("postprocessing_template_ids", [])
-                if pp_ids and pp_store:
-                    pool = ImagePool()
-                    for pp_id in pp_ids:
-                        try:
-                            pp = pp_store.get_template(pp_id)
-                            for fi in pp_store.resolve_filter_instances(pp.filters):
-                                f = FilterRegistry.create_instance(fi.filter_id, fi.options)
-                                result = f.process_image(cur_image, pool)
-                                if result is not None:
-                                    cur_image = result
-                        except Exception:
-                            pass
+                # NOTE: postprocessing is already applied by the ProcessedLiveStream
+                # when using LiveStreamManager.acquire() — do NOT re-apply here.
 
                 # Re-read source for hot-update support
                 try:
@@ -647,11 +641,10 @@ async def test_key_colors_ws(
     except Exception as e:
         logger.error(f"KC CSS test WS error: {e}", exc_info=True)
     finally:
-        if live_stream is not None:
-            try:
-                await asyncio.to_thread(live_stream_mgr.release, source.picture_source_id)
-            except Exception:
-                pass
+        try:
+            await asyncio.to_thread(live_stream_mgr.release, source.picture_source_id)
+        except Exception:
+            pass
 
 
 # ===== CALIBRATION TEST =====