fix(mpc-service): enable shutdown hooks for graceful Kafka disconnect

Root cause: When mpc-service restarts, the old Kafka consumer doesn't
properly disconnect, causing the broker to wait for sessionTimeout
(~5 minutes) before completing rebalance. This blocks app startup.

Solution: Enable NestJS shutdown hooks with app.enableShutdownHooks().
This ensures onModuleDestroy() is called on SIGTERM/SIGINT, which calls
consumer.disconnect() and allows immediate rebalance on next startup.

Also reverted the "don't await consumer.run()" workaround since the
proper fix is graceful shutdown.

Sources:
- https://github.com/tulios/kafkajs/issues/807
- https://kafka.js.org/docs/consuming

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

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

backend/services/mpc-service/src/application/services/event-consumer-starter.service.ts

@@ -16,11 +16,7 @@ export class EventConsumerStarterService implements OnApplicationBootstrap {
 
   async onApplicationBootstrap() {
     try {
-      // Don't await - consumer.run() never resolves (it runs continuously)
-      // Just fire and forget, let it run in the background
-      this.eventConsumer.startConsuming().catch((error) => {
-        this.logger.error('Kafka consumer error', error);
-      });
+      await this.eventConsumer.startConsuming();
       this.logger.log('MPC event consumers started successfully');
     } catch (error) {
       this.logger.error('Failed to start MPC event consumers', error);

backend/services/mpc-service/src/main.ts

@@ -18,6 +18,10 @@ async function bootstrap() {
     logger: ['error', 'warn', 'log', 'debug', 'verbose'],
   });
 
+  // Enable graceful shutdown hooks (calls onModuleDestroy on SIGTERM/SIGINT)
+  // This ensures Kafka consumer disconnects properly, allowing fast rebalance on restart
+  app.enableShutdownHooks();
+
   // Get config service
   const configService = app.get(ConfigService);
   const port = parseInt(process.env.APP_PORT || '3006', 10);