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

frontend/src/types/index.ts

@@ -28,6 +28,7 @@ export interface Application {
   port: number;
   userId: string;
   clusterId?: string;
+  poolId?: string;
   latestImageTag?: string;
   subdomain?: string;
   deployments?: Deployment[];
@@ -95,6 +96,37 @@ export interface CreateApplicationDto {
   memoryLimit?: string;
   replicas?: number;
   port?: number;
+  clusterId?: string;
+  poolId?: string;
+}
+
+export interface ClusterPublic {
+  id: string;
+  name: string;
+  region?: string;
+  provider?: string;
+  isDefault: boolean;
+  status: 'active' | 'inactive' | 'maintenance';
+}
+
+export interface ClusterPoolPublic {
+  id: string;
+  name: string;
+  description?: string;
+  strategy: 'least-apps' | 'round-robin';
+  clusterIds: string[];
+  isActive: boolean;
+  clusters: Pick<ClusterPublic, 'id' | 'name' | 'region' | 'provider' | 'status'>[];
+}
+
+export interface ClusterPool {
+  id: string;
+  name: string;
+  description?: string;
+  strategy: 'least-apps' | 'round-robin';
+  clusterIds: string[];
+  isActive: boolean;
+  createdAt: string;
 }
 
 export interface PodInfo {