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-customer-accounts/tasks.md

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+## 1. Account persistence and mutation boundary
+
+- [ ] 1.1 Add additive migrations for customer accounts, account kinds/currencies, append-only entries, and required indexes/checks.
+- [ ] 1.2 Initialize zero account rows for existing seeded users and every newly registered customer. Do not credit demo points here.
+- [ ] 1.3 Implement the Rust account module with stable-order account locking, conditional non-negative debits/freezes, atomic credits/releases, and entry insertion.
+
+## 2. Summary contract and proof
+
+- [ ] 2.1 Add shared account-summary types, client method, localized UI strings, and fixed-data adapter compatibility.
+- [ ] 2.2 Expose authenticated `GET /api/me/stats` through the account service without public balance mutation endpoints.
+- [ ] 2.3 Add API integration tests for zero initialization, summary ownership, immutable entries, available-to-frozen transfer, and competing insufficient debits.
+
+## 3. Mall migration and verification
+
+- [ ] 3.1 Replace buyer-center and points-page `USER_STATS` reads with the shared account summary API.
+- [ ] 3.2 Remove affected stats fixture imports while keeping the fixed-data adapter rollback path intact.
+- [ ] 3.3 Seed and browser-smoke the live buyer account summary.
+- [ ] 3.4 Run cargo test for vmall-api, the mall build, and strict validation for this OpenSpec change.