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/op25_client.py

@@ -65,6 +65,16 @@ class OP25Client:
             logger.error(f"OP25 status failed: {e}")
             return None
 
+    async def devices(self) -> Optional[Dict[str, Any]]:
+        try:
+            async with httpx.AsyncClient(timeout=5) as client:
+                r = await client.get(f"{self.api_url}/op25/devices")
+                r.raise_for_status()
+                return r.json()
+        except Exception as e:
+            logger.error(f"OP25 device enumeration failed: {e}")
+            return None
+
     async def generate_config(self, config: Dict[str, Any]) -> bool:
         try:
             async with httpx.AsyncClient(timeout=10) as client: