feat: multi-cluster management with pool-based load balancing

- Add ClusterPool entity for grouping clusters into named pools
- Support 3 deployment modes: manual cluster, pool load-balanced, default fallback
- Pool strategies: least-apps (fewest deployed apps) and round-robin
- Add pool CRUD API endpoints (admin) and public pool listing
- Frontend deploy page: 3-mode cluster selector (Default/Manual/Pool)
- Frontend app detail: shows assigned cluster and pool info
- Admin pools management page with cluster selection and strategy picker
- Application entity extended with poolId field

backend/src/clusters/clusters.module.ts

@@ -3,9 +3,10 @@ import { TypeOrmModule } from '@nestjs/typeorm';
 import { ClustersService } from './clusters.service';
 import { ClustersController } from './clusters.controller';
 import { Cluster } from './entities/cluster.entity';
+import { ClusterPool } from './entities/cluster-pool.entity';
 
 @Module({
-  imports: [TypeOrmModule.forFeature([Cluster])],
+  imports: [TypeOrmModule.forFeature([Cluster, ClusterPool])],
   controllers: [ClustersController],
   providers: [ClustersService],
   exports: [ClustersService],