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/lib/stockNotifications.ts

@@ -0,0 +1,25 @@
+// Server-only — imported exclusively by app/api/stock-notifications/route.ts.
+// Reuses ORDER_SERVICE_SECRET rather than a new dedicated secret, matching
+// StockNotifications.ts's own choice on the backend (see that collection's
+// comment) — it's already the general-purpose "frontend server calling
+// into Payload" credential used by getWishlistEnabled/getSearchEnabled/etc.
+const PAYLOAD_URL = process.env.PAYLOAD_URL || "https://payload.mk360.de";
+const SERVICE_SECRET = process.env.ORDER_SERVICE_SECRET || "";
+
+export async function createStockNotification(email: string, productId: number, variantName: string): Promise<{ ok: boolean; reason?: string }> {
+  const res = await fetch(`${PAYLOAD_URL}/api/stock-notifications`, {
+    method: "POST",
+    headers: { "Content-Type": "application/json", "x-order-service-secret": SERVICE_SECRET },
+    body: JSON.stringify({ email, product: productId, variantName }),
+  });
+  if (res.ok) return { ok: true };
+
+  // The (tenant, email, product, variantName) unique index rejects a
+  // second signup for the same thing with a 400 — read as "already on the
+  // list", not a real error, so the UI can still show success rather than
+  // a confusing failure for someone who's already signed up.
+  if (res.status === 400) return { ok: true };
+
+  console.error(`createStockNotification: Payload returned ${res.status} ${res.statusText}`);
+  return { ok: false, reason: "Eintragen hat nicht geklappt. Bitte versuch es später erneut." };
+}