Fix SMS billing build and make credit conversion idempotent.

Import the purchase-price constant for Nest build, and only convert IRT-scale balances so migrate re-runs do not divide credits again.

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

src/auth/sms-billing.service.ts

@@ -1,7 +1,7 @@
 import { BadRequestException, Injectable, NotFoundException } from '@nestjs/common';
 import { PrismaService } from '../prisma/prisma.service';
 import { TenantService } from '../tenant/tenant.service';
-import { calculateSmsCredits, irtToSmsCredits } from './sms-billing';
+import { calculateSmsCredits, irtToSmsCredits, SMS_PURCHASE_PRICE_PER_CREDIT_IRT } from './sms-billing';
 
 @Injectable()
 export class SmsBillingService {