fix(mpc-service): reduce rebalanceTimeout from 5min to 1min

sessionTimeout remains at 5 minutes (needed for long MPC operations),
but rebalanceTimeout only affects joining consumer group, not processing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/mpc-service/src/infrastructure/messaging/kafka/event-consumer.service.ts

@@ -70,7 +70,7 @@ export class EventConsumerService implements OnModuleInit, OnModuleDestroy {
       sessionTimeout: 300000, // 5 minutes - MPC keygen can take a while
       heartbeatInterval: 10000, // 10 seconds
       maxWaitTimeInMs: 5000,
-      rebalanceTimeout: 300000, // 5 minutes
+      rebalanceTimeout: 60000, // 1 minute - just for joining group, not for processing
     });
 
     try {