fix: send calendar heartbeat as post

plugin/calendar/calendar-bridge.ts

@@ -14,7 +14,7 @@
  *   Default backendUrl: "https://monitor.hangman-lab.top"
  *
  * Endpoints used:
- *   GET  /calendar/agent/heartbeat        — fetch pending slots
+ *   POST /calendar/agent/heartbeat        — fetch pending slots
  *   PATCH /calendar/slots/{id}/agent-update   — update real slot status
  *   PATCH /calendar/slots/virtual/{vid}/agent-update — update virtual slot status
  *
@@ -77,7 +77,7 @@ export class CalendarBridgeClient {
 
     try {
       const response = await this.fetchJson<CalendarHeartbeatResponse>(url, {
-        method: 'GET',
+        method: 'POST',
         headers: {
           'Content-Type': 'application/json',
           'X-Agent-ID': this.config.agentId,