Add second-SDR plumbing: secondary_sdr_mode config + hardware reporting

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>

op25-container/Dockerfile

@@ -12,7 +12,7 @@ ENV DEBIAN_FRONTEND=noninteractive
 # Install system dependencies
 RUN apt-get update && \
     apt-get upgrade -y && \
-    apt-get install git pulseaudio pulseaudio-utils liquidsoap -y
+    apt-get install git pulseaudio pulseaudio-utils liquidsoap usbutils -y
 
 # Install custom PulseAudio system config (enables anonymous access for edge-node)
 COPY system.pa /etc/pulse/system.pa