Add prepaid resource credits with prorated deploy billing.

When users delete an app before plan expiry, remaining resources become credits for a new deploy. The deploy calculator shows covered vs additional charges, prices optional services correctly, and prorates extras to days left on the credit.

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

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

@@ -143,6 +143,12 @@ export class CalculateCostDto {
   enableCustomDomain?: boolean;
 }
 
+export class CalculateDeployCostDto extends CalculateCostDto {
+  @ApiProperty({ enum: BillingCycle, example: 'monthly' })
+  @IsEnum(BillingCycle)
+  cycle: BillingCycle;
+}
+
 // ─── Renewal & Upgrade DTOs ─────────────────────────────────────────
 
 export class RenewApplicationDto {