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.

openspec/changes/add-group-buying/tasks.md

@@ -1,14 +1,14 @@
 ## 1. Group-buying state model and contract
 
-- [ ] 1.1 Add additive migrations for shop activities, concrete groups, paid memberships, expiry/state constraints, and order group-price snapshots.
-- [ ] 1.2 Implement the Rust group-buying module with shop-scoped activity management, active discovery, deterministic expiry, and open-group queries.
+- [x] 1.1 Add additive migrations for shop activities, concrete groups, paid memberships, expiry/state constraints, and order group-price snapshots.
+- [x] 1.2 Implement the Rust group-buying module with shop-scoped activity management, active discovery, deterministic expiry, and open-group queries.
 - [ ] 1.3 Add shared group-buying types, checkout intent (single SKU, quantity 1), API-client methods, locales, and fixed-data adapter parity.
 
 ## 2. Checkout and payment integration
 
-- [ ] 2.1 Validate a single-SKU quantity-1 group-buying intent during checkout, create or reference an open group, and snapshot group price and identity on the pending order. Reject overlapping flash eligibility and coupon selection on that shop order; restate coupon and flash handling for other shop orders in the same checkout.
-- [ ] 2.2 Extend payment to atomically expire due groups, claim a paid seat, enforce capacity, and transition a filled group to successful. Pending-payment cancellation restores SKU stock only and does not roll back paid membership. Cancelling the opening unpaid order while paid count is zero marks the group cancelled.
-- [ ] 2.3 Add integration tests for activity/shop validation, open versus join flow, quantity-1 enforcement, concurrent final-seat payment, full/expired/cancelled-group conflicts, coupon rejection on group shop orders, overlapping flash rejection, cancel-without-membership-rollback, unpaid-opener empty-group cancel, and visibility of expired paid groups.
+- [x] 2.1 Validate a single-SKU quantity-1 group-buying intent during checkout, create or reference an open group, and snapshot group price and identity on the pending order. Reject overlapping flash eligibility and coupon selection on that shop order; restate coupon and flash handling for other shop orders in the same checkout.
+- [x] 2.2 Extend payment to atomically expire due groups, claim a paid seat, enforce capacity, and transition a filled group to successful. Pending-payment cancellation restores SKU stock only and does not roll back paid membership. Cancelling the opening unpaid order while paid count is zero marks the group cancelled.
+- [x] 2.3 Add integration tests for activity/shop validation, open versus join flow, quantity-1 enforcement, concurrent final-seat payment, full/expired/cancelled-group conflicts, coupon rejection on group shop orders, overlapping flash rejection, cancel-without-membership-rollback, unpaid-opener empty-group cancel, and visibility of expired paid groups.
 
 ## 3. Merchant and customer surfaces
 
@@ -19,4 +19,4 @@
 ## 4. Verification and specification
 
 - [ ] 4.1 Seed an active deterministic group activity and browser-smoke discovery, opening, joining, and successful payment.
-- [ ] 4.2 Run cargo test for vmall-api, builds for mall and shop-admin, and strict validation for this OpenSpec change.
+- [ ] 4.2 Run cargo test for vmall-api, builds for mall and shop-admin, and strict validation for this OpenSpec change.