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/dto/cluster-pool.dto.ts

@@ -0,0 +1,50 @@
+import { IsString, IsOptional, IsBoolean, IsArray, IsIn } from 'class-validator';
+import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
+
+export class CreateClusterPoolDto {
+  @ApiProperty({ example: 'production-pool' })
+  @IsString()
+  name: string;
+
+  @ApiPropertyOptional({ example: 'Load-balanced pool for production workloads' })
+  @IsOptional()
+  @IsString()
+  description?: string;
+
+  @ApiProperty({ example: 'least-apps', enum: ['least-apps', 'round-robin'] })
+  @IsIn(['least-apps', 'round-robin'])
+  strategy: 'least-apps' | 'round-robin';
+
+  @ApiProperty({ example: ['uuid-1', 'uuid-2'], description: 'Array of cluster IDs in this pool' })
+  @IsArray()
+  @IsString({ each: true })
+  clusterIds: string[];
+}
+
+export class UpdateClusterPoolDto {
+  @ApiPropertyOptional({ example: 'production-pool-v2' })
+  @IsOptional()
+  @IsString()
+  name?: string;
+
+  @ApiPropertyOptional()
+  @IsOptional()
+  @IsString()
+  description?: string;
+
+  @ApiPropertyOptional({ enum: ['least-apps', 'round-robin'] })
+  @IsOptional()
+  @IsIn(['least-apps', 'round-robin'])
+  strategy?: 'least-apps' | 'round-robin';
+
+  @ApiPropertyOptional({ description: 'Array of cluster IDs in this pool' })
+  @IsOptional()
+  @IsArray()
+  @IsString({ each: true })
+  clusterIds?: string[];
+
+  @ApiPropertyOptional()
+  @IsOptional()
+  @IsBoolean()
+  isActive?: boolean;
+}