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/applications/entities/application.entity.ts

@@ -74,6 +74,9 @@ export class Application {
   @Column({ nullable: true })
   clusterId: string;
 
+  @Column({ nullable: true })
+  poolId: string; // Cluster pool used for load-balanced assignment
+
   @OneToMany(() => Deployment, (deployment: Deployment) => deployment.application)
   deployments: Deployment[];