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/storage/template_store.py

@@ -8,7 +8,7 @@ from typing import Dict, List, Optional
 
 from wled_controller.core.capture_engines.factory import EngineRegistry
 from wled_controller.storage.template import CaptureTemplate
-from wled_controller.utils import get_logger
+from wled_controller.utils import atomic_write_json, get_logger
 
 logger = get_logger(__name__)
 
@@ -95,23 +95,14 @@ class TemplateStore:
     def _save(self) -> None:
         """Save all templates to file."""
         try:
-            # Ensure directory exists
-            self.file_path.parent.mkdir(parents=True, exist_ok=True)
-
-            templates_dict = {
-                template_id: template.to_dict()
-                for template_id, template in self._templates.items()
-            }
-
             data = {
                 "version": "1.0.0",
-                "templates": templates_dict,
+                "templates": {
+                    template_id: template.to_dict()
+                    for template_id, template in self._templates.items()
+                },
             }
-
-            # Write to file
-            with open(self.file_path, "w", encoding="utf-8") as f:
-                json.dump(data, f, indent=2, ensure_ascii=False)
-
+            atomic_write_json(self.file_path, data)
         except Exception as e:
             logger.error(f"Failed to save templates to {self.file_path}: {e}")
             raise
@@ -218,6 +209,9 @@ class TemplateStore:
 
         # Update fields
         if name is not None:
+            for tid, t in self._templates.items():
+                if tid != template_id and t.name == name:
+                    raise ValueError(f"Template with name '{name}' already exists")
             template.name = name
         if engine_type is not None:
             template.engine_type = engine_type