Stripe Payment Processing & Subscriptions

graph TD
    User[Candidate Dashboard] -->|Click Upgrade| Route[Next.js API: Initialize Checkout Session]
    Route -->|Call Stripe API| StripeCheckout[Redirect to Stripe Checkout Page]
    StripeCheckout -->|User Pays & Completes| RedirectSuccess[Redirect to /payment/success]
    StripeCheckout -->|User Cancels| RedirectCancel[Redirect to /payment/cancel]
    
    StripeCheckout -->|Async Event: checkout.session.completed| Webhook[Stripe Webhook Endpoint]
    Webhook -->|Verify Signature| UpdateDB[Update Firestore user tier: premium/pro]

import { NextRequest, NextResponse } from "next/server";
import { auth } from "@clerk/nextjs/server";
import Stripe from "stripe";

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!, {
  apiVersion: "2023-10-16" as any,
});

export async function POST(request: NextRequest) {
  try {
    const { userId } = await auth();
    if (!userId) {
      return new NextResponse("Unauthorized request", { status: 401 });
    }

    const { priceId } = await request.json();
    if (!priceId) {
      return NextResponse.json({ error: "Missing priceId parameter" }, { status: 400 });
    }

    // Initialize Stripe Checkout session
    const session = await stripe.checkout.sessions.create({
      payment_method_types: ["card"],
      line_items: [
        {
          price: priceId,
          quantity: 1,
        },
      ],
      mode: "subscription",
      success_url: `${process.env.NEXT_PUBLIC_APP_URL}/payment/success?session_id={CHECKOUT_SESSION_ID}`,
      cancel_url: `${process.env.NEXT_PUBLIC_APP_URL}/payment/cancel`,
      metadata: {
        userId: userId, // Pass Clerk ID to match in webhook callback
      },
    });

    return NextResponse.json({ url: session.url });

  } catch (error) {
    console.error("Failed to initialize checkout session:", error);
    return NextResponse.json({ error: "Checkout session initialization failed" }, { status: 500 });
  }
}