feat: log backfill endpoint for desktop log panel

- LogStreamService: ring buffer (500 entries) + getRecentLogs() method
- SupervisorController: GET /api/supervisor/logs/recent returns buffered
  log entries so the desktop log panel shows history on tab open, not
  just live stream

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

src/supervisor/supervisor.controller.ts

@@ -78,6 +78,11 @@ export class SupervisorController {
         );
     }
 
+    @Get('logs/recent')
+    getRecentLogs(@Query('limit') limit?: string) {
+        return LogStreamService.instance.getRecentLogs(limit ? parseInt(limit, 10) : 200);
+    }
+
     @Sse('logs/stream')
     streamLogs(): Observable<MessageEvent> {
         this.logger.log('[SSE] Log stream opened');