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/billing.module.ts

@@ -7,13 +7,14 @@ import { PricingRule } from './entities/pricing-rule.entity';
 import { Wallet } from './entities/wallet.entity';
 import { WalletTransaction } from './entities/wallet-transaction.entity';
 import { PlatformSetting } from './entities/platform-setting.entity';
+import { ResourceCredit } from './entities/resource-credit.entity';
 import { LifecycleModule } from '../lifecycle/lifecycle.module';
 import { ApplicationsModule } from '../applications/applications.module';
 import { KubernetesModule } from '../kubernetes/kubernetes.module';
 
 @Module({
   imports: [
-    TypeOrmModule.forFeature([ServicePlan, PricingRule, Wallet, WalletTransaction, PlatformSetting]),
+    TypeOrmModule.forFeature([ServicePlan, PricingRule, Wallet, WalletTransaction, PlatformSetting, ResourceCredit]),
     forwardRef(() => LifecycleModule),
     forwardRef(() => ApplicationsModule),
     forwardRef(() => KubernetesModule),