Add second-SDR fields: secondary_sdr_mode + sdr_count on NodeRecord

Server-side half of node-26#9. NodeRecord gains secondary_sdr_mode
(none|adsb|ais|op25_2) and sdr_count; checkin ingestion stores both,
and PATCH /nodes/{id} accepts and re-pushes secondary_sdr_mode the same
way hardware_preset/ppm_override already work, so it isn't wiped by a
system reassignment (see server-26#111 for the pre-existing bug that
pattern avoids repeating).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

drb-c2-core/app/internal/mqtt_handler.py

@@ -111,6 +111,8 @@ class MQTTHandler:
                 "assigned_system_id": None,
                 "approval_status": "pending",
                 "node_type": payload.get("node_type", "fixed"),
+                "secondary_sdr_mode": payload.get("secondary_sdr_mode", "none"),
+                "sdr_count": payload.get("sdr_count", 1),
                 "enforce_override_timeout": payload.get("enforce_override_timeout", True),
                 "is_overridden": False,
                 "override_system_id": None,
@@ -141,6 +143,11 @@ class MQTTHandler:
             updates["node_type"] = node_type
             updates["enforce_override_timeout"] = enforce_timeout
 
+            if "secondary_sdr_mode" in payload:
+                updates["secondary_sdr_mode"] = payload["secondary_sdr_mode"]
+            if "sdr_count" in payload:
+                updates["sdr_count"] = payload["sdr_count"]
+
             if node_type == "portable":
                 updates["is_overridden"] = False
                 updates["override_system_id"] = None