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-frontend/lib/c2api.ts

@@ -35,7 +35,10 @@ export const c2api = {
     request(`/nodes/${nodeId}/override/ack`, { method: "POST", body: JSON.stringify({ timeout_minutes: timeoutMinutes }) }),
   resetOverride: (nodeId: string) =>
     request(`/nodes/${nodeId}/override/reset`, { method: "POST" }),
-  updateNode: (id: string, body: { node_type?: string; enforce_override_timeout?: boolean }) =>
+  updateNode: (
+    id: string,
+    body: { node_type?: string; enforce_override_timeout?: boolean; secondary_sdr_priority?: string[] },
+  ) =>
     request(`/nodes/${id}`, { method: "PATCH", body: JSON.stringify(body) }),
 
   // Systems