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/dto/application.dto.ts

@@ -88,6 +88,11 @@ export class CreateApplicationDto {
   @IsOptional()
   @IsString()
   clusterId?: string;
+
+  @ApiPropertyOptional({ description: 'Cluster pool ID for load-balanced deployment' })
+  @IsOptional()
+  @IsString()
+  poolId?: string;
 }
 
 export class UpdateApplicationDto {