Optimize WLED processing pipeline and add FPS metrics

- Add numpy-based DDP pixel packing (send_pixels_numpy) and fast send
  path (send_pixels_fast) eliminating per-pixel Python loops
- Move ProcessedLiveStream filter processing to background thread so
  get_latest_frame() returns pre-computed cached result instantly
- Vectorize map_border_to_leds for average interpolation using cumulative
  sums instead of 934 individual np.mean calls (~16ms -> <1ms)
- Batch all CPU work into single asyncio.to_thread call per frame
- Fix FPS calculation to measure frame-to-frame interval (was measuring
  processing time only, reporting inflated values)
- Add Potential FPS metric showing theoretical max without throttling
- Add FPS label to WLED target card properties
- Add fps_potential field to TargetProcessingState API schema

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

server/src/wled_controller/static/locales/en.json

@@ -130,6 +130,7 @@
   "device.stopped": "Processing stopped",
   "device.metrics.actual_fps": "Actual FPS",
   "device.metrics.target_fps": "Target FPS",
+  "device.metrics.potential_fps": "Potential FPS",
   "device.metrics.frames": "Frames",
   "device.metrics.errors": "Errors",
   "device.health.online": "WLED Online",