Integrate Stripe Payments for Pro Tier Upgrades

[Sumedh-6504]

Description

Implement Stripe subscription integration so that users can upgrade their account status from "free" to "pro" ($5/month) directly from the /plans page.

User Story

As a LexGuard Free user,
I want to upgrade my account to the Pro tier via a secure payment interface,
So that I can unlock unlimited analyses and full 3-agent worst-case scenarios.

Technical Tasks

• Install @stripe/stripe-js on the Next.js frontend and stripe SDK on the backend.
• Create a secure Next.js API endpoint /api/billing/checkout to generate a Stripe Checkout session.
• Wire the "Upgrade to Pro" buttons on /plans to redirect the user to the Stripe Checkout page.
• Create a robust Stripe webhook handler at /api/webhooks/stripe to handle events:
  • checkout.session.completed -> Update user plan to "pro" in Supabase.
  • customer.subscription.deleted -> Revert user plan to "free" in Supabase.
• Store the customer's Stripe Customer ID in a new column on the users table to maintain billing history.

Database Updates

ALTER TABLE users ADD COLUMN IF NOT EXISTS stripe_customer_id TEXT;

[hipfeed]

I would tighten the webhook side before wiring the upgrade buttons. A safe Pro upgrade flow should make the database row the source of truth, not the Checkout return URL.

Useful checks to add to this issue:

1. preserve the raw request body before stripe.webhooks.constructEvent
2. store Stripe event IDs so duplicate deliveries cannot grant Pro twice or create inconsistent state
3. map checkout.session.completed to a local subscription/customer row, then derive users.plan from that durable state
4. handle customer.subscription.updated and invoice.payment_failed, not only deleted
5. fail closed if the webhook secret or price ID is missing in production

That keeps /plans simple while making the paid entitlement recoverable if Stripe retries or the browser never returns from Checkout.

[Sumedh-6504]

Thank you for your help! 👍, will work on the webhook tightening on database.