feat(api): group buying with payment-time seat claims

Shop-owned activities on one SKU, concrete groups with an
open/successful/expired/cancelled lifecycle, and one paid membership row per
paid order. Checkout accepts a single-SKU quantity-1 intent, snapshots the group
price and identity on the pending order, and opens or references a group; a paid
seat is claimed only at payment, which locks the group and fills it exactly at
capacity.

Pending-payment cancellation restores SKU stock only and never rolls back paid
seats; an unpaid opener cancelling closes a still-empty group. Coupons are
refused on a group shop order, and the flash-sale exclusion is now enforced in
both directions because the activity table exists, which activates the guard
add-flash-sales shipped dormant. The activity column names follow the contract
recorded in this change's design.

Surfaces (shop-admin, mall) and seeding follow.

apps/api/src/modules/mod.rs

@@ -8,6 +8,7 @@ pub mod coupon;
 pub mod currency;
 pub mod flash_sale;
 pub mod fulfillment;
+pub mod group_buying;
 pub mod health;
 pub mod identity;
 pub mod order;
@@ -30,6 +31,7 @@ pub fn api_router() -> Router<AppState> {
         .merge(cart::router())
         .merge(coupon::router())
         .merge(flash_sale::router())
+        .merge(group_buying::router())
         .merge(order::router())
         .merge(points::router())
         .merge(shop::router())