feat: real-time active call SSE — hold/unhold status for supervisor live monitor

- SupervisorService: added activeCallSubject (RxJS Subject), emits on all
  activeCalls Map mutations (Answered, Calling, Disconnect, Hold, Unhold)
- SupervisorController: new @Sse('active-calls/stream') endpoint
- OzonetelAgentController: callControl HOLD/UNHOLD updates activeCalls Map
  status via supervisor.updateCallStatus()

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/supervisor/supervisor.controller.ts

@@ -13,6 +13,16 @@ export class SupervisorController {
         return this.supervisor.getActiveCalls();
     }
 
+    @Sse('active-calls/stream')
+    streamActiveCalls(): Observable<MessageEvent> {
+        this.logger.log('[SSE] Active calls stream opened');
+        return this.supervisor.activeCallSubject.pipe(
+            map(event => ({
+                data: JSON.stringify(event),
+            } as MessageEvent)),
+        );
+    }
+
     @Get('team-performance')
     async getTeamPerformance(@Query('date') date?: string) {
         const targetDate = date ?? new Date().toISOString().split('T')[0];