Revamp Trial Subscription Flow with Stripe SetupIntent, Legacy Support, and Webhook Fixes

[Ashad001]

This pull request fixes critical issues in the subscription system and implements a robust, modern trial flow using Stripe SetupIntents. It also adds backward compatibility for legacy subscriptions and improves webhook signature verification reliability.

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🔍 Issues Addressed

Issue #1: Premature Invoice Creation

• Problem: Stripe subscription and invoice were created immediately when the user visited the checkout page.
• Fix: Modified the /api/checkout-sessions endpoint to only create a SetupIntent (not a subscription). The subscription is now created after payment confirmation via the /api/trial/start endpoint.
• Impact: No invoices are generated until the user confirms payment.

Issue #2: Legacy Users Can't Cancel

• Problem: Legacy users had stripeSubscriptionId values like pi_... (Payment Intent), not actual sub_... (Subscription IDs), causing cancellation to fail.

• Fix: Added logic in /api/trial/cancel to detect and gracefully handle legacy users:

  • If the ID starts with pi_, it skips Stripe API cancellation and cleans up Redis instead.

• Impact: Legacy users can now cancel without issues.

Issue #3: Webhook Signature Failures

• Problem: Stripe webhook verification was failing despite the secret being configured.

• Fix:

  • Improved error handling and diagnostics in /api/webhooks
  • Added clearer logging for missing/invalid secrets and signature mismatches
  • Ensured correct raw body usage to support Stripe signature verification

• Impact: Easier debugging and better observability of webhook behavior.

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Major Changes

• New Trial Flow:

  • Users first create a SetupIntent on the checkout page.

  • After confirming the payment method, /api/trial/start is called to:

    • Create the Stripe subscription
    • Add credits (500) to Redis

  • No invoice is generated until this step is complete.

• Legacy Support:

  • Detects and handles legacy users with pi_-based IDs
  • Avoids Stripe errors by skipping subscription API calls for such users

• Frontend Updates:

  • Updated CheckoutForm to align with the new SetupIntent-first flow
  • Adjusted state handling in /checkout/page.tsx to support the new API
  • Updated UI logic to reflect new subscription and trial state handling