feat(webhooks): outbound webhooks with HMAC-SHA256 signing and API key auth

Webhook registration/management. HMAC-signed delivery with retry.
Events: recipe.created/updated, comment.created, follower.new.
REST API key creation for programmatic access.

apps/web/app/api/webhooks/stripe/route.ts

@@ -0,0 +1,19 @@
+import { NextRequest, NextResponse } from "next/server";
+
+// Stripe webhook stub — wire up when adding Stripe
+// Verifies stripe-signature header (using raw body), handles:
+// - checkout.session.completed → upgrade user to pro
+// - customer.subscription.deleted → downgrade user to free
+export async function POST(req: NextRequest) {
+  const body = await req.text();
+  const sig = req.headers.get("stripe-signature");
+
+  if (!sig || !process.env["STRIPE_WEBHOOK_SECRET"]) {
+    return NextResponse.json({ error: "Stripe not configured" }, { status: 400 });
+  }
+
+  // TODO: const event = stripe.webhooks.constructEvent(body, sig, process.env["STRIPE_WEBHOOK_SECRET"]);
+  // For now just return 200 to acknowledge receipt
+  console.log("[stripe-webhook] received event, sig:", sig.slice(0, 20));
+  return NextResponse.json({ received: true });
+}