Handle incoming Stripe webhook events.

This endpoint receives webhook events from Stripe for:

• checkout.session.completed: Successful one-time payments
• invoice.paid: Successful subscription/invoice payments
• invoice.payment_failed: Failed payment attempts

The endpoint:

1. Verifies the webhook signature to ensure authenticity
2. Processes the event based on its type
3. Stores changes in the database
4. Returns 200 only AFTER database commit (per user requirement)

Note: This endpoint is public but protected by Stripe signature verification. Only requests signed with the correct webhook secret will be processed.