Add back-in-stock notification signup form

NotifyMeForm.tsx replaces the disabled Add-to-cart button's spot once
a product/variant is out of stock — POSTs to the new
/api/stock-notifications route, which forwards to the backend's new
stock-notifications collection. Threaded product.numericId (not the
commerce slug id) into AddToCartButton/AddToCartInlineButton for this,
same split WishlistButton already uses.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/api/stock-notifications/route.ts

@@ -0,0 +1,21 @@
+import { NextResponse } from "next/server";
+import { isValidEmail } from "../../lib/email";
+import { createStockNotification } from "../../lib/stockNotifications";
+
+export async function POST(request: Request) {
+  const body = await request.json().catch(() => null);
+  const email = typeof body?.email === "string" ? body.email.trim() : "";
+  const productId = Number(body?.productId);
+  const variantName = typeof body?.variantName === "string" ? body.variantName : "";
+
+  if (!isValidEmail(email)) {
+    return NextResponse.json({ ok: false, reason: "Bitte gib eine gültige E-Mail-Adresse ein." }, { status: 400 });
+  }
+  if (!Number.isInteger(productId) || productId <= 0) {
+    return NextResponse.json({ ok: false, reason: "Ungültiges Produkt." }, { status: 400 });
+  }
+
+  const result = await createStockNotification(email, productId, variantName);
+  if (!result.ok) return NextResponse.json(result, { status: 500 });
+  return NextResponse.json({ ok: true });
+}