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/dto/application.dto.ts

@@ -32,6 +32,16 @@ export class CreateApplicationDto {
   @IsEnum(DatabaseType)
   databaseType: DatabaseType;
 
+  @ApiPropertyOptional({ example: 'appuser', description: 'Database username (default: appuser)' })
+  @IsOptional()
+  @IsString()
+  dbUsername?: string;
+
+  @ApiPropertyOptional({ example: 'mySecurePass123', description: 'Database password (auto-generated if empty)' })
+  @IsOptional()
+  @IsString()
+  dbPassword?: string;
+
   @ApiPropertyOptional({ example: 'https://github.com/user/repo.git' })
   @IsOptional()
   @IsString()