Optimize KC processing and add reactive HAOS state updates

- Optimize KC frame processing: downsample to 160x90 with cv2.resize
  before rectangle extraction, pre-compute pixel coords, vectorize
  smoothing with numpy arrays
- Add WebSocket event stream for server state changes: processor manager
  fires events on start/stop, new /api/v1/events/ws endpoint streams
  them to connected clients
- Add HAOS EventStreamListener that triggers coordinator refresh on
  state changes for near-instant switch updates
- Reduce HAOS polling interval from 10s to 3s for fresher FPS metrics
- Fix overlay button tooltips: flatten nested JSON keys in locale files
  to match flat dot-notation lookup used by t() function

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

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

@@ -791,6 +791,44 @@ async def target_colors_ws(
         manager.remove_kc_ws_client(target_id, websocket)
 
 
+# ===== STATE CHANGE EVENT STREAM =====
+
+
+@router.websocket("/api/v1/events/ws")
+async def events_ws(
+    websocket: WebSocket,
+    token: str = Query(""),
+):
+    """WebSocket for real-time state change events. Auth via ?token=<api_key>."""
+    authenticated = False
+    cfg = get_config()
+    if token and cfg.auth.api_keys:
+        for _label, api_key in cfg.auth.api_keys.items():
+            if secrets.compare_digest(token, api_key):
+                authenticated = True
+                break
+
+    if not authenticated:
+        await websocket.close(code=4001, reason="Unauthorized")
+        return
+
+    await websocket.accept()
+
+    manager = get_processor_manager()
+    queue = manager.subscribe_events()
+
+    try:
+        while True:
+            event = await queue.get()
+            await websocket.send_json(event)
+    except WebSocketDisconnect:
+        pass
+    except Exception:
+        pass
+    finally:
+        manager.unsubscribe_events(queue)
+
+
 # ===== OVERLAY VISUALIZATION =====
 
 @router.post("/api/v1/picture-targets/{target_id}/overlay/start", tags=["Visualization"])