Make the matrix frame rate configurable

24 fps costs about 11% of one core on a Zero 2 W, measured on the board. That
is a reasonable default for something you look at, but it is a decorative load
sharing a 512 MB machine with the forecaster, so it should be the owner's
choice. server.led_fps is clamped to 4..30.

Particle fall speed now divides by the configured rate rather than the module
constant, so rain falls at the same real-world speed whatever the frame rate,
instead of slowing down when you turn the frame rate down.

ashvale/api.py

@@ -60,7 +60,7 @@ async def lifespan(app: FastAPI):
     station.sample_once()
     station.start()
     if CONFIG.server.led_enabled:
-        display = LedDisplay(station, CONFIG.server.led_cycle_s)
+        display = LedDisplay(station, CONFIG.server.led_cycle_s, CONFIG.server.led_fps)
         display.start()
     try:
         yield