Closes #218

Description

This Pull Request overhauls the PaymentsModule by replacing the placeholder mock provider with a fully operational real-world Stripe API integration for teachLink_backend.

The backend can now handle processing live payment intents, correctly tracking/processing refunds, and fully verifying incoming asynchronous payment webhook events securely.

Specifications

• Added environment schema validations for STRIPE_SECRET_KEY & STRIPE_WEBHOOK_SECRET.
• Enabled rawBody: true globally via NestFactory to allow accurate Stripe webhook cryptographic signature verification.
• Implemented actual Stripe SDK pm/pi creations and refund dispatching within StripeService.
• Replaced the hard-coded mocked getProvider method inside PaymentsService with a dynamic ProviderFactoryService injection.
• Refactored Webhook handling to capture raw buffer streams securely through NestJS and process payment_intent.succeeded/failed natively.

Close #[issue-number]

Acceptance Criteria Achieved

• Real payments processed through Stripe
• Refunds processed correctly
• Webhooks securely handle payment events and signatures
• Payment failures handled gracefully
• Transaction history accurately kept

Note: Please make sure to add the new Stripe secret keys to your environment configurations (e.g. .env) locally and upstream before testing 🚀!

close issue #218
close issue #219