feat: add custom domain support with SSL, DNS verification, and billing

Users can assign a custom domain to their app with automatic SSL via
cert-manager. Includes DNS verification flow (CNAME check), Persian
instructions, admin-configurable pricing via PlatformSetting, and
integration into the deploy wizard cost calculation.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/billing/dto/billing.dto.ts

@@ -136,6 +136,11 @@ export class CalculateCostDto {
   @IsOptional()
   @IsBoolean()
   enableElasticsearch?: boolean;
+
+  @ApiPropertyOptional({ example: false, description: 'Enable custom domain with SSL' })
+  @IsOptional()
+  @IsBoolean()
+  enableCustomDomain?: boolean;
 }
 
 // ─── Renewal & Upgrade DTOs ─────────────────────────────────────────