docs(openspec): plan shop coupons, accounts, points, flash sales, and group buying

Capture exclusive promotion composition, empty-group cancel, and mall surface scope so the five marketing changes can be implemented without stacking or lifecycle ambiguity.

Co-authored-by: Cursor <cursoragent@cursor.com>

openspec/changes/add-flash-sales/specs/frontend-shop-admin/spec.md

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+## ADDED Requirements
+
+### Requirement: Merchant flash-sale management
+Shop users SHALL manage only their flash-sale sessions and items in shop-admin, choosing their own SKU, active window, minor-unit sale price, reserved stock, and per-customer limit. Shop-admin SHALL expose a flash-sale navigation entry beside existing shop operations.
+
+#### Scenario: configure item
+- **WHEN** a merchant creates an active valid flash-sale item for its SKU
+- **THEN** it appears to customers through active flash-sale discovery
+
+#### Scenario: flash sales appear in shop navigation
+- **WHEN** an authenticated shop user opens shop-admin
+- **THEN** a flash-sale management entry is reachable without leaving the shop-scoped console