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/routers/nodes.py

@@ -195,6 +195,7 @@ async def assign_system(
 class NodeUpdateBody(BaseModel):
     node_type: Optional[str] = None
     enforce_override_timeout: Optional[bool] = None
+    secondary_sdr_mode: Optional[str] = None  # none | adsb | ais | op25_2
 
 
 @router.patch("/{node_id}")
@@ -227,6 +228,8 @@ async def update_node(
             }
             if updated_node.get("ppm_override") is not None:
                 push_payload["ppm_override"] = updated_node["ppm_override"]
+            if updated_node.get("secondary_sdr_mode") is not None:
+                push_payload["secondary_sdr_mode"] = updated_node["secondary_sdr_mode"]
             mqtt_handler.push_config(node_id, push_payload)
 
     return {"ok": True}