## Why

The mall's flash-sale page is display-only and prices are fixture-derived. A real event must allocate stock and resolve activity pricing inside checkout, not trust a price shown by the client.

## What Changes

- Add shop-managed flash-sale sessions and activity products with localized display data, fixed minor-unit activity price, window, per-customer limit, and isolated activity stock.
- Add public session and product discovery APIs and replace the flash-sale page fixtures with live data.
- Make checkout resolve an active eligible activity server-side, atomically decrement activity stock with SKU stock, and snapshot the applied activity price on the order item.
- Compose exclusively with other promotions: a line MAY receive flash pricing or group-buy pricing, never both; a shop order that applied any flash price SHALL reject a coupon. Restate shop-coupon redemption and pending-payment coupon restore in this change's `order` spec so archival does not drop them.
- Add shop-admin session and activity-product management scoped to the authenticated shop; mall payment and order-detail views show the snapshotted activity price.

## Capabilities

### New Capabilities
- `flash-sales`: Timed shop flash-sale sessions, reserved activity inventory, eligibility, and public discovery.

### Modified Capabilities
- `order`: Checkout resolves and snapshots active flash-sale pricing.
- `frontend-mall`: Flash-sale discovery and product purchase use live data.
- `frontend-shop-admin`: Shops manage their own flash-sale configuration.

## Non-goals

Queues, bot mitigation, platform-wide campaigns, waitlists, automatic retry after sell-out, and stacking coupons onto flash-priced shop orders are excluded. Overlapping group-buy on the same SKU is rejected rather than merged.

## Impact

Adds a Rust marketing module and migrations, shared contract/locales, mall and shop-admin screens, and checkout concurrency coverage.