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>

drb-edge-node/app/main.py

@@ -224,6 +224,7 @@ async def on_config_push(payload: dict):
     hardware_preset = payload.pop("hardware_preset", None)
     ppm_override = payload.pop("ppm_override", None)
     node_type = payload.pop("node_type", None)
+    secondary_sdr_mode = payload.pop("secondary_sdr_mode", None)
     enforce_override_timeout = payload.pop("enforce_override_timeout", None)
     try:
         config = SystemConfig(**payload)
@@ -243,6 +244,8 @@ async def on_config_push(payload: dict):
         node_cfg.ppm_override = float(ppm_override)
     if node_type is not None:
         node_cfg.node_type = node_type
+    if secondary_sdr_mode is not None:
+        node_cfg.secondary_sdr_mode = secondary_sdr_mode
     if enforce_override_timeout is not None:
         node_cfg.enforce_override_timeout = bool(enforce_override_timeout)
     save_node_config(node_cfg)