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/core/devices/ddp_client.py

@@ -193,12 +193,13 @@ class DDPClient:
         try:
             # Send plain RGB — WLED handles per-bus color order conversion
             # internally when outputting to hardware.
+            # Convert to numpy to avoid per-pixel Python loop
             bpp = 4 if self.rgbw else 3  # bytes per pixel
-            pixel_bytes = bytearray()
-            for r, g, b in pixels:
-                pixel_bytes.extend((int(r), int(g), int(b)))
-                if self.rgbw:
-                    pixel_bytes.append(0)  # White channel = 0
+            pixel_array = np.array(pixels, dtype=np.uint8)
+            if self.rgbw:
+                white = np.zeros((pixel_array.shape[0], 1), dtype=np.uint8)
+                pixel_array = np.hstack((pixel_array, white))
+            pixel_bytes = pixel_array.tobytes()
 
             total_bytes = len(pixel_bytes)
             # Align payload to full pixels (multiple of bpp) to avoid splitting