chore(seed): seed an active group-buying activity

Add one active activity for the demo shop on a SKU that is not already in the
seeded flash sale, idempotent by name, so the live mall can open and join a
group.

scripts/seed-demo.mjs

@@ -409,6 +409,50 @@ const POINTS_PRODUCTS = [
   console.log("flash sale ready");
 }
 
+// 8. an active group-buying activity for the demo shop (idempotent by name).
+{
+  const owner = ownerTokens.get("demo-store");
+  const listed = await call("GET", "/shop/group-buying-activities", { token: owner });
+  if (listed.status !== 200) fail("list group activities", listed);
+  const known = new Set(listed.data.map((a) => a.name.en));
+  if (!known.has("Weekend group deal")) {
+    const products = await call("GET", "/shop/products?per_page=100", { token: owner });
+    if (products.status !== 200) fail("list shop products", products);
+    const flash = await call("GET", "/shop/flash-sales", { token: owner });
+    const flashSkus = new Set(
+      (flash.data ?? []).flatMap((s) => s.items ?? []).map((i) => i.sku_id),
+    );
+    const used = new Set(listed.data.map((a) => a.sku_id));
+    // A SKU already in an overlapping flash sale cannot join a group activity.
+    const sku = (products.data.items ?? [])
+      .flatMap((p) => p.skus ?? [])
+      .find((s) => s.active && s.stock > 0 && !flashSkus.has(s.id) && !used.has(s.id));
+    if (!sku) fail("find a group-buying SKU", { status: 0, data: "no free SKU" });
+
+    const now = Date.now();
+    const res = await call("POST", "/shop/group-buying-activities", {
+      token: owner,
+      body: {
+        sku_id: sku.id,
+        name: { en: "Weekend group deal", zh: "周末拼团" },
+        description: {
+          en: "Gather two friends to unlock the group price.",
+          zh: "两人成团，享拼团价。",
+        },
+        group_price_minor: Math.max(1, Math.round(sku.price_minor * 0.75)),
+        currency: sku.currency,
+        required_members: 2,
+        starts_at: new Date(now - 3600 * 1000).toISOString(),
+        ends_at: new Date(now + 7 * 24 * 3600 * 1000).toISOString(),
+        group_lifetime_hours: 24,
+        enabled: true,
+      },
+    });
+    if (res.status !== 201) fail("seed group activity", res);
+  }
+  console.log("group buying ready");
+}
+
 console.log("\nDemo data seeded.");
 console.log("  platform admin : admin@vmall.local / admin1234   (apps/admin :3002)");
 console.log("  shop owner     : shop@vmall.local / shop12345    (apps/shop-admin :3001)");