Next.js / Stripe / Webhook Debugging + Production Launch

I'm looking for a senior Next.js backend developer to fully debug and finalize my production website.

Stack
Next.js 15
App Router
TypeScript
Stripe Checkout
Stripe Webhooks
Vercel
Supplier API (Romania)
Google Tag Manager
Current issue

The payment flow is:

Customer
↓
Stripe Checkout
↓
Stripe Payment
↓
Stripe Webhook
↓
Detect operator
↓
View services
↓
PlaceOrder
↓
ConfirmOrder

Everything works until ConfirmOrder.

Current behavior
✅ Stripe payment succeeds
✅ Webhook executes
✅ Operator detection works
✅ Service lookup works
✅ PlaceOrder returns status_code = 0
✅ order_uid is received
❌ ConfirmOrder returns status_code = 4

The mobile credit is not delivered.

What has already been verified
Stripe Checkout
Stripe Webhooks
Metadata
Operator detection
Service lookup
Environment variables
Multiple deployments
Multiple rollbacks
I need someone to:
Debug the complete payment flow.
Find the root cause of the ConfirmOrder failure.
Verify Stripe webhook implementation.
Verify Casier Total API integration.
Verify request hashing/signatures.
Compare working vs current deployment if necessary.
Make the project fully functional end-to-end.
Configure the production domain correctly (all redirects, environment variables, callbacks, Stripe URLs, etc.).
Perform a successful real payment test and confirm the entire purchase flow works.
Install and configure Google Tag Manager (GTM).
Make sure the production deployment is ready for launch.
Expected result

When the project is finished:

✅ Production domain correctly configured
✅ Stripe working
✅ Webhooks working
✅ Casier Total delivers mobile credit successfully
✅ Successful real payment completed
✅ GTM installed
✅ Production-ready deployment