feat(frontend): route checkout through V2 Identity plan-purchase; drop dead Stripe webhook

/api/checkout now resolves the requested plan (pro|business × monthly|annual)
to a plan GUID via gateway /v1/plans (codes follow pro_monthly / business_annual)
and POSTs /v1/users/me/plan/purchase. The payments service owns the gateway
(ZarinPal/Stripe) and returns redirect_url, which we hand back unchanged.

Removes the orphaned Stripe→Supabase webhook + lib/stripe.ts client: profile
plan reads come from Identity now, so the Supabase profiles upsert loop is dead.
V2 payments has its own gateway callback (skip-auth, payment-callback route).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/stripe.ts

@@ -1,20 +0,0 @@
-import Stripe from "stripe";
-
-let stripeClient: Stripe | null = null;
-
-export function getStripe(): Stripe {
-  if (!stripeClient) {
-    const secretKey = process.env.STRIPE_SECRET_KEY;
-
-    if (!secretKey) {
-      throw new Error("Missing STRIPE_SECRET_KEY environment variable.");
-    }
-
-    stripeClient = new Stripe(secretKey, {
-      apiVersion: "2026-04-22.dahlia",
-      typescript: true,
-    });
-  }
-
-  return stripeClient;
-}