feat(education): save practice answers idempotently

sql/mysql/education/003-education-answer-idempotency-rollback.sql

@@ -0,0 +1,30 @@
+-- =============================================
+-- Education 模块 — 答案保存幂等性回滚
+-- Ticket #7 / Migration 003
+-- =============================================
+--
+-- WARNING: This file contains NO executable SQL.
+-- Destructive rollback requires manual operator verification.
+--
+-- Manual rollback procedure (operator must execute):
+-- 1. Verify no other tables depend on education_answer_idempotency:
+--      SELECT TABLE_NAME, COLUMN_NAME, REFERENCED_TABLE_NAME
+--        FROM information_schema.KEY_COLUMN_USAGE
+--       WHERE REFERENCED_TABLE_NAME = 'education_answer_idempotency'
+--         AND TABLE_SCHEMA = DATABASE();
+--    Result MUST be empty before proceeding.
+--
+-- 2. Verify the table contains only data from this migration:
+--      SELECT COUNT(*) AS idempotency_count FROM education_answer_idempotency;
+--    Operator must confirm this count is acceptable to destroy.
+--
+-- 3. Verify no application code depends on client_sequence column:
+--    Search codebase for 'clientSequence' / 'client_sequence' references.
+--
+-- 4. After verification, execute:
+--      DROP TABLE IF EXISTS education_answer_idempotency;
+--      ALTER TABLE education_practice_question DROP COLUMN client_sequence;
+--
+-- DO NOT uncomment or execute the lines below without operator verification.
+-- -- DROP TABLE IF EXISTS education_answer_idempotency;
+-- -- ALTER TABLE education_practice_question DROP COLUMN client_sequence;