Adds a per-node secondary_sdr_mode config field (none|adsb|ais|op25_2), applied the same way as hardware_preset/ppm_override so it survives system reassignment. op25-container gets a GET /devices endpoint that counts connected SDRs via lsusb; the edge-node checkin now reports sdr_count and secondary_sdr_mode up to the server, the first node-initiated hardware report (everything else was C2 pushing config down). Tracked as node-26#9. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>