feat: extract shop from mp/shop — initial libreshop/shop

Source moved verbatim from mp/shop/ on 2026-04-29; mp was the first
concrete adapter consuming the libreshop toolkit. Builds and publishes
git.librete.ch/libreshop/shop on every main / v* push via the standard
.gitea/workflows/build.yml shared across libreshop components.

server/api/orders/[uuid]/checkout.post.ts

@@ -0,0 +1,16 @@
+import { fetchCms } from "~/server/utils/cmsApi";
+
+export default defineEventHandler(async (event) => {
+	const uuid = getRouterParam(event, "uuid");
+	const query = getQuery(event);
+
+	if (!uuid) {
+		throw createError({ statusCode: 400, statusMessage: "Missing order UUID" });
+	}
+
+	const returnUrl = query.returnUrl || "";
+
+	return await fetchCms(`/orders/${uuid}/checkout?returnUrl=${encodeURIComponent(String(returnUrl))}`, {
+		method: "POST"
+	});
+});