Codebase review fixes: stability, performance, quality improvements

Stability: Add outer try/except/finally with _running=False cleanup to all 6
processing loop methods (live, color_strip, effect, audio, composite, mapped).
Add exponential backoff on consecutive capture errors in live_stream. Move
audio stream.stop() outside lock scope.

Performance: Replace per-pixel Python loop with np.array().tobytes() in
ddp_client. Vectorize pixelate filter with cv2.resize down+up. Vectorize
gradient rendering with np.searchsorted.

Frontend: Add lockBody/unlockBody re-entrancy counter. Add {once:true} to
fetchWithAuth abort listener. Null ws.onclose before ws.close() in LED preview.

Backend: Remove auth token prefix from log messages. Add atomic_write_json
helper (tempfile + os.replace) and update all 10 stores. Add name uniqueness
checks to all update methods. Fix DELETE status codes to 204 in audio_sources
and value_sources. Fix get_source() silent bug in color_strip_sources.

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

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

@@ -129,7 +129,7 @@ async def stream_capture_test(
             done_event.set()
 
     # Start capture in background thread
-    loop = asyncio.get_event_loop()
+    loop = asyncio.get_running_loop()
     capture_future = loop.run_in_executor(None, _capture_loop)
 
     start_time = time.perf_counter()
@@ -142,6 +142,8 @@ async def stream_capture_test(
 
             # Check for init error
             if init_error:
+                stop_event.set()
+                await capture_future
                 await websocket.send_json({"type": "error", "detail": init_error})
                 return