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/value_sources.py

@@ -23,6 +23,7 @@ from wled_controller.storage.value_source_store import ValueSourceStore
 from wled_controller.storage.output_target_store import OutputTargetStore
 from wled_controller.core.processing.processor_manager import ProcessorManager
 from wled_controller.utils import get_logger
+from wled_controller.storage.base_store import EntityNotFoundError
 
 logger = get_logger(__name__)
 
@@ -105,6 +106,9 @@ async def create_value_source(
         )
         fire_entity_event("value_source", "created", source.id)
         return _to_response(source)
+    except EntityNotFoundError as e:
+        raise HTTPException(status_code=404, detail=str(e))
+
     except ValueError as e:
         raise HTTPException(status_code=400, detail=str(e))
 
@@ -158,6 +162,9 @@ async def update_value_source(
         pm.update_value_source(source_id)
         fire_entity_event("value_source", "updated", source_id)
         return _to_response(source)
+    except EntityNotFoundError as e:
+        raise HTTPException(status_code=404, detail=str(e))
+
     except ValueError as e:
         raise HTTPException(status_code=400, detail=str(e))
 
@@ -182,6 +189,9 @@ async def delete_value_source(
 
         store.delete_source(source_id)
         fire_entity_event("value_source", "deleted", source_id)
+    except EntityNotFoundError as e:
+        raise HTTPException(status_code=404, detail=str(e))
+
     except ValueError as e:
         raise HTTPException(status_code=400, detail=str(e))