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/applications/applications.service.ts

@@ -163,7 +163,7 @@ export class ApplicationsService {
 
   async delete(id: string, userId: string): Promise<Application> {
     const app = await this.findOne(id, userId);
-    
+
     // Delete uploaded files
     if (app.codePath) {
       try {