Backend performance and code quality improvements

Performance (hot path):
- Fix double brightness: removed duplicate scaling from 9 device clients
  (wled, adalight, ambiled, openrgb, hue, spi, chroma, gamesense, usbhid,
  espnow) — processor loop is now the single source of brightness
- Bounded send_timestamps deque with maxlen, removed 3 cleanup loops
- Running FPS sum O(1) instead of sum()/len() O(n) per frame
- datetime.now(timezone.utc) → time.monotonic() with lazy conversion
- Device info refresh interval 30 → 300 iterations
- Composite: gate layer_snapshots copy on preview client flag
- Composite: versioned sub_streams snapshot (copy only on change)
- Composite: pre-resolved blend methods (dict lookup vs getattr)
- ApiInput: np.copyto in-place instead of astype allocation

Code quality:
- BaseJsonStore: RLock on get/delete/get_all/count (was created but unused)
- EntityNotFoundError → proper 404 responses across 15 route files
- Remove 21 defensive getattr(x,'tags',[]) — field guaranteed on all models
- Fix Dict[str,any] → Dict[str,Any] in template/audio_template stores
- Log 4 silenced exceptions (automation engine, metrics, system)
- ValueStream.get_value() now @abstractmethod
- Config.from_yaml: add encoding="utf-8"
- OutputTargetStore: remove 25-line _load override, use _legacy_json_keys
- BaseJsonStore: add _legacy_json_keys for migration support
- Remove unnecessary except Exception→500 from postprocessing list endpoint

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -68,6 +68,7 @@ psutil.cpu_percent(interval=None)
 
 # GPU monitoring (initialized once in utils.gpu, shared with metrics_history)
 from wled_controller.utils.gpu import nvml_available as _nvml_available, nvml as _nvml, nvml_handle as _nvml_handle
+from wled_controller.storage.base_store import EntityNotFoundError
 
 
 def _get_cpu_name() -> str | None:
@@ -96,8 +97,8 @@ def _get_cpu_name() -> str | None:
                 .decode()
                 .strip()
             )
-    except Exception:
-        pass
+    except Exception as e:
+        logger.warning("CPU name detection failed: %s", e)
     return platform.processor() or None
 
 
@@ -157,7 +158,7 @@ async def list_all_tags(_: AuthRequired):
         items = fn() if fn else None
         if items:
             for item in items:
-                all_tags.update(getattr(item, 'tags', []))
+                all_tags.update(item.tags)
     return {"tags": sorted(all_tags)}
 
 
@@ -205,6 +206,10 @@ async def get_displays(
             count=len(displays),
         )
 
+    except EntityNotFoundError as e:
+        raise HTTPException(status_code=404, detail=str(e))
+
+
     except ValueError as e:
         raise HTTPException(status_code=400, detail=str(e))
     except Exception as e:
@@ -265,8 +270,8 @@ def get_system_performance(_: AuthRequired):
                 memory_total_mb=round(mem_info.total / 1024 / 1024, 1),
                 temperature_c=float(temp),
             )
-        except Exception:
-            pass
+        except Exception as e:
+            logger.debug("NVML query failed: %s", e)
 
     return PerformanceResponse(
         cpu_name=_cpu_name,