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/internal/mqtt_manager.py

@@ -213,7 +213,9 @@ class MQTTManager:
     async def _publish_checkin(self):
         from app.internal.discord_radio import radio_bot
         from app.internal.config_manager import load_node_config
+        from app.internal.op25_client import op25_client
         config = load_node_config()
+        devices = await op25_client.devices()
         payload = {
             "node_id": settings.node_id,
             "name": settings.node_name,
@@ -225,7 +227,12 @@ class MQTTManager:
             "is_overridden": config.override_system_id is not None and config.node_type != "portable",
             "override_system_id": config.override_system_id,
             "enforce_override_timeout": config.enforce_override_timeout,
+            "secondary_sdr_mode": config.secondary_sdr_mode,
         }
+        # Best-effort — the first node-initiated hardware-report field. Omit
+        # rather than guess if op25's control API is unreachable.
+        if devices is not None:
+            payload["sdr_count"] = devices.get("count")
         self._publish(self._t_checkin, payload, qos=1)
 
     def _publish(self, topic: str, payload: dict, qos: int = 0, retain: bool = False):