feat: database management — custom credentials, dump upload/restore

Backend:
- Add dbUsername/dbPassword columns to Application entity
- Add optional DB credential fields to CreateApplicationDto
- Auto-generate dbPassword (crypto.randomBytes) and default dbUsername='appuser'
  when databaseType != 'none' on app creation
- Store both username and password in K8s DB secret (was password-only)
- Read DB_USER/POSTGRES_USER/MYSQL_USER from secretKeyRef instead of hardcoded
- New restoreDatabaseDump() in KubernetesService: creates K8s Job with
  psql/mysql client to restore uploaded SQL dump, waits for completion,
  returns logs
- New POST /applications/:id/db-upload endpoint with 500MB file limit

Frontend:
- Add dbUsername/dbPassword to Application and CreateApplicationDto types
- Deploy page: show username/password fields when database is selected,
  with generate-random-password button and show/hide toggle
- App detail page: new Database section with connection info (host, port,
  db name, username, password with copy-to-clipboard), SQL dump upload
  area with drag-and-drop, and restore output logs display

Security:
- Database remains ClusterIP only (no external exposure)
- Credentials stored in K8s Secrets (base64-encoded)
- Dump file uploaded as temporary K8s Secret, auto-cleaned after restore

backend/src/applications/applications.service.ts

@@ -4,10 +4,11 @@ import { Repository } from 'typeorm';
 import { ConfigService } from '@nestjs/config';
 import * as fs from 'fs';
 import * as path from 'path';
+import * as crypto from 'crypto';
 import { Application } from './entities/application.entity';
 import { CreateApplicationDto, UpdateApplicationDto } from './dto/application.dto';
 import { ClustersService } from '../clusters/clusters.service';
-import { UserRole } from '../common/enums';
+import { UserRole, DatabaseType } from '../common/enums';
 
 @Injectable()
 export class ApplicationsService {
@@ -66,11 +67,22 @@ export class ApplicationsService {
       this.logger.log(`Manual cluster assignment for app "${dto.name}" → cluster ${clusterId}`);
     }
 
+    // Generate database credentials if a database is requested
+    let dbUsername: string | undefined;
+    let dbPassword: string | undefined;
+    if (dto.databaseType && dto.databaseType !== DatabaseType.NONE) {
+      dbUsername = dto.dbUsername?.trim() || 'appuser';
+      dbPassword = dto.dbPassword?.trim() || crypto.randomBytes(16).toString('hex');
+      this.logger.log(`Generated DB credentials for app "${dto.name}" — user: ${dbUsername}`);
+    }
+
     const app = this.appsRepository.create({
       ...dto,
       userId,
       clusterId,
       poolId,
+      dbUsername,
+      dbPassword,
       subdomain: `${dto.name}-${userId.split('-')[0]}`,
     });
     return this.appsRepository.save(app);