The "Plot path to beacon" action now includes `tx_power_dbm` and
`src_gain_dbi` in the /path query string so the path calculator's TX
side emits the same EIRP as the beacon out of the box. The beacon
schema stores `power_mw` as EIRP already (per Beacons.RangeEstimate),
so we take 10·log10(power_mw), split against a default 30 dBi dish,
and clamp the TX power to a 0 dBm floor so very-low-EIRP beacons
produce a sane gain/power split instead of -10 dBm outputs.
Examples:
10 W EIRP (40 dBm) → 30 dBi dish + 10 dBm TX
100 W EIRP (50 dBm) → 30 dBi dish + 20 dBm TX
100 mW EIRP (20 dBm) → 20 dBi + 0 dBm (clamped)