feat(mall): claim and redeem shop coupons from the storefront

The product page loads a shop's claimable coupons through the shared client and
claims them in place; the buyer coupon list reads owned snapshots with their
claimed/redeemed/expired state. Checkout offers each shop order at most one
owned coupon and submits only the choice; the pay page and order detail render
the server-persisted discount and reduced total, never a client figure.

Coupon fixtures leave the pages; the fixed-data adapter still serves the coupon
domain as the rollback path. The demo seed creates two deterministic templates
and claims them for the demo customer.

scripts/seed-demo.mjs

@@ -173,6 +173,34 @@ for (const def of SHOPS) {
   ownerTokens.set(def.slug, await ensureShop(def));
 }
 
+// 2b. shop coupons: deterministic, idempotent by localized title.
+const DEMO_COUPONS = [
+  { title: { en: "$5 off over $59", zh: "满 59 减 5" }, amount_minor: 500, threshold_minor: 5900, stock: 200 },
+  { title: { en: "$15 off over $199", zh: "满 199 减 15" }, amount_minor: 1500, threshold_minor: 19900, stock: 100 },
+];
+let demoShopId = "";
+{
+  const owner = ownerTokens.get("demo-store");
+  const profile = await call("GET", "/shop/profile", { token: owner });
+  if (profile.status !== 200) fail("coupon shop profile", profile);
+  demoShopId = profile.data.id;
+
+  const listed = await call("GET", "/shop/coupon-templates", { token: owner });
+  if (listed.status !== 200) fail("list coupon templates", listed);
+  const known = new Set(listed.data.map((t) => t.title.en));
+  const starts_at = new Date(Date.now() - 24 * 3600 * 1000).toISOString();
+  const ends_at = new Date(Date.now() + 365 * 24 * 3600 * 1000).toISOString();
+  for (const coupon of DEMO_COUPONS) {
+    if (known.has(coupon.title.en)) continue;
+    const res = await call("POST", "/shop/coupon-templates", {
+      token: owner,
+      body: { ...coupon, currency: "USD", enabled: true, starts_at, ends_at },
+    });
+    if (res.status !== 201) fail(`seed coupon ${coupon.title.en}`, res);
+  }
+  console.log(`coupons ready: ${DEMO_COUPONS.length}`);
+}
+
 // 3. demo customer
 await call("POST", "/auth/register", {
   body: { email: "customer@vmall.local", password: "customer123", display_name: "Demo Customer" },
@@ -194,6 +222,19 @@ await call("POST", "/auth/register", {
       if (r.status !== 200 && r.status !== 201) fail("seed address", r);
     }
   }
+
+  // Claim the demo shop's coupons so the buyer starts with live ones.
+  const claimable = (await call("GET", `/shops/${demoShopId}/coupon-templates`)).data;
+  if (Array.isArray(claimable)) {
+    for (const template of claimable) {
+      const res = await call("POST", "/me/coupons", {
+        token: customerToken,
+        body: { template_id: template.id },
+      });
+      // 409 means a previous seed run already claimed it.
+      if (res.status !== 201 && res.status !== 409) fail(`claim coupon ${template.id}`, res);
+    }
+  }
 }
 
 // 4. categories (reference data from the migrations)