Fix FPS drops caused by brightness endpoint polling WLED device

The GET /devices/{id}/brightness endpoint was making an HTTP request to
the ESP32 over WiFi on every frontend poll (~3s), causing 150ms async
event loop jitter that froze the LED processing loop. Cache brightness
server-side after first fetch/set, add frontend dedup guard, reduce
get_device_info() frequency, and add processing loop timing diagnostics.

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

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

@@ -331,16 +331,29 @@ async def get_device_brightness(
     store: DeviceStore = Depends(get_device_store),
     manager: ProcessorManager = Depends(get_processor_manager),
 ):
-    """Get current brightness from the device."""
+    """Get current brightness from the device.
+
+    Uses a server-side cache to avoid polling the physical device on every
+    frontend request — hitting the ESP32 over WiFi in the async event loop
+    causes ~150 ms jitter in the processing loop.
+    """
     device = store.get_device(device_id)
     if not device:
         raise HTTPException(status_code=404, detail=f"Device {device_id} not found")
     if "brightness_control" not in get_device_capabilities(device.device_type):
         raise HTTPException(status_code=400, detail=f"Brightness control is not supported for {device.device_type} devices")
 
+    # Return cached hardware brightness if available (updated by SET endpoint)
+    ds = manager._devices.get(device_id)
+    if ds and ds.hardware_brightness is not None:
+        return {"brightness": ds.hardware_brightness}
+
     try:
         provider = get_provider(device.device_type)
         bri = await provider.get_brightness(device.url)
+        # Cache the result so subsequent polls don't hit the device
+        if ds:
+            ds.hardware_brightness = bri
         return {"brightness": bri}
     except NotImplementedError:
         # Provider has no hardware brightness; use software brightness
@@ -381,8 +394,12 @@ async def set_device_brightness(
             if device_id in manager._devices:
                 manager._devices[device_id].software_brightness = bri
 
-        # If device is idle with a static color, re-send it at the new brightness
+        # Update cached hardware brightness
         ds = manager._devices.get(device_id)
+        if ds:
+            ds.hardware_brightness = bri
+
+        # If device is idle with a static color, re-send it at the new brightness
         if ds and ds.static_color is not None and not manager.is_device_processing(device_id):
             try:
                 await manager.send_static_color(device_id, ds.static_color)