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/static/js/features/devices.js

@@ -288,7 +288,10 @@ export async function saveCardBrightness(deviceId, value) {
     }
 }
 
+const _brightnessFetchInFlight = new Set();
 export async function fetchDeviceBrightness(deviceId) {
+    if (_brightnessFetchInFlight.has(deviceId)) return;
+    _brightnessFetchInFlight.add(deviceId);
     try {
         const resp = await fetch(`${API_BASE}/devices/${deviceId}/brightness`, {
             headers: getHeaders()
@@ -306,6 +309,8 @@ export async function fetchDeviceBrightness(deviceId) {
         if (wrap) wrap.classList.remove('brightness-loading');
     } catch (err) {
         // Silently fail — device may be offline
+    } finally {
+        _brightnessFetchInFlight.delete(deviceId);
     }
 }