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/applications.service.ts

@@ -20,22 +20,46 @@ export class ApplicationsService {
   ) {}
 
   async create(userId: string, dto: CreateApplicationDto): Promise<Application> {
-    // Auto-assign default cluster if not specified
+    // Cluster assignment: 3 modes
+    // 1. Manual: specific clusterId provided
+    // 2. Pool-based LB: poolId provided → pick from pool using pool's strategy
+    // 3. Default fallback: no clusterId/poolId → use default cluster
+
     let clusterId = dto.clusterId;
+    let poolId = dto.poolId;
+
     if (!clusterId) {
-      try {
-        const defaultCluster = await this.clustersService.getDefault();
-        clusterId = defaultCluster.id;
-        this.logger.log(`Auto-assigned default cluster "${defaultCluster.name}" to app "${dto.name}"`);
-      } catch {
-        this.logger.warn('No default cluster found — app will be created without cluster assignment');
+      if (poolId) {
+        // Mode 2: Pool-based load balancing
+        try {
+          const optimal = await this.clustersService.getOptimalClusterFromPool(poolId);
+          clusterId = optimal.id;
+          this.logger.log(`Pool-assigned cluster "${optimal.name}" to app "${dto.name}" (pool LB)`);
+        } catch (err: any) {
+          this.logger.warn(`Pool assignment failed for app "${dto.name}": ${err.message}`);
+          poolId = undefined; // Clear invalid pool
+        }
       }
+
+      // Mode 3: Default fallback (no manual cluster, no pool, or pool failed)
+      if (!clusterId) {
+        try {
+          const defaultCluster = await this.clustersService.getDefault();
+          clusterId = defaultCluster.id;
+          this.logger.log(`Default-assigned cluster "${defaultCluster.name}" to app "${dto.name}"`);
+        } catch {
+          this.logger.warn('No cluster available — app will be created without cluster assignment');
+        }
+      }
+    } else {
+      this.logger.log(`Manual cluster assignment for app "${dto.name}" → cluster ${clusterId}`);
     }
 
     const app = this.appsRepository.create({
       ...dto,
       userId,
       clusterId,
+      poolId,
       subdomain: `${dto.name}-${userId.split('-')[0]}`,
     });
     return this.appsRepository.save(app);

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 {

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[];