Secondary SDR priority: C2 field, node command, dashboard section

Pairs with node-26 feat/secondary-sdr-priority. NodeRecord gains
secondary_sdr_priority (ordered; SDRs beyond OP25's run it top-down) and
secondary_sdr_running, both mirrored from the node's checkin.
PATCH /nodes/{id} accepts the priority, validates it, and sends it as a
'set_secondary_priority' MQTT command. A priority-only change never
re-pushes system config, because that restarts OP25. The node detail page
gets a 'Secondary SDRs' section (admin-editable) with enable, reorder,
save, and live Running / Waiting-for-SDR state from the checkin.

Verified: c2-core pytest 482 passed; frontend tsc --noEmit clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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

@@ -112,6 +112,8 @@ class MQTTHandler:
                 "approval_status": "pending",
                 "node_type": payload.get("node_type", "fixed"),
                 "secondary_sdr_mode": payload.get("secondary_sdr_mode", "none"),
+                "secondary_sdr_priority": payload.get("secondary_sdr_priority", []),
+                "secondary_sdr_running": payload.get("secondary_sdr_running"),
                 "sdr_count": payload.get("sdr_count", 1),
                 "enforce_override_timeout": payload.get("enforce_override_timeout", True),
                 "is_overridden": False,
@@ -143,8 +145,9 @@ 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"]
+            for key in ("secondary_sdr_mode", "secondary_sdr_priority", "secondary_sdr_running"):
+                if key in payload:
+                    updates[key] = payload[key]
             if "sdr_count" in payload:
                 updates["sdr_count"] = payload["sdr_count"]