Subscribe to "propagation:updated" on connected mount and, when the
grid worker finishes a forecast-hour step, re-read point_forecast for
the path midpoint using the stashed band_mhz and source/destination.
Terrain, HRRR profiles, and the loss/power budget are independent of
grid scores and stay as-is. Before this, a /path page rendered once at
mount and only refreshed on manual reload.
