feat(education): add question favorites

sql/mysql/education/007-education-favorite-rollback.sql

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+-- =============================================
+-- Education 模块 — 收藏夹 DDL Rollback
+-- Migration: 007
+-- =============================================
+-- IMPORTANT: This is a documentation-only rollback.
+--   No DROP/ALTER/DELETE statements are executed. The favorite
+--   table is provenance-safe: it only accumulates user preference
+--   data. Dropping this table would lose student favorites with
+--   no recovery path.
+--
+-- What this migration created:
+--   - education_favorite (new table)
+--
+-- Manual rollback requires:
+--   1. Verified database backup before rollback
+--   2. Operator approval (DBA sign-off)
+--   3. Provenance of all favorite records preserved (exported)
+--   4. Soft-delete via deleted = b'1' before any hard drop
+--
+-- These tables are NOT deleted by this script. Favorite history is
+-- retained; if deletion is required by external policy, consult
+-- the DBA for a verified rollback procedure.

sql/mysql/education/007-education-favorite.sql

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+-- =============================================
+-- Education 模块 — 收藏夹 DDL
+-- Ticket #10: 学生收藏题目
+-- Migration: 007
+-- Prerequisites: 000-education-schema.sql (base tables)
+-- =============================================
+
+-- =============================================
+-- Preconditions
+-- =============================================
+-- Operator is expected to verify:
+--   SELECT COUNT(*) FROM information_schema.tables
+--     WHERE table_schema = DATABASE()
+--       AND table_name = 'education_favorite';
+-- Result MUST be 0 before executing this migration.
+
+-- =============================================
+-- 收藏表
+-- =============================================
+-- Purpose: Student favorites for questions with safe snapshots.
+--   Each (tenant, user, target_type, target_id) is a unique entry.
+--   Logical deletion: setting deleted=1 marks as unfavorited.
+--   Re-adding after deletion reactivates the row via ON DUPLICATE KEY UPDATE.
+--
+-- target_type values: 'QUESTION' (extensible enum)
+--
+-- Snapshot fields (stem, type, difficulty, options, content_version):
+--   populated at creation time from the visible question's safe fields.
+--   These snapshots preserve the question state as it appeared when favorited,
+--   and remain stable even if the source question later changes or becomes unavailable.
+--
+-- available flag:
+--   FALSE when the source question becomes hidden/unpublished after being
+--   favorited. Existing favorites with available=FALSE remain listable but
+--   display an "unavailable" indicator. New favorites cannot be created for
+--   unavailable resources.
+--
+-- Indexes:
+--   uk_tenant_user_target — per (tenant, user, target_type, target_id) uniqueness.
+--     INSERT ... ON DUPLICATE KEY UPDATE is the primary reactivation path.
+--   idx_tenant_user — covers listing queries filtered by current tenant+user.
+--   idx_tenant_user_target_type — covers target-type-filtered listing.
+CREATE TABLE `education_favorite` (
+    `id`               BIGINT       NOT NULL AUTO_INCREMENT COMMENT '主键',
+    `tenant_id`        BIGINT       NOT NULL              COMMENT '租户编号',
+    `user_id`          BIGINT       NOT NULL              COMMENT '学生用户编号',
+    `target_type`      VARCHAR(32)  NOT NULL              COMMENT '目标类型：QUESTION',
+    `target_id`        VARCHAR(64)  NOT NULL              COMMENT '目标 ID（题目 ID）',
+    -- safe snapshot fields
+    `stem`             TEXT         DEFAULT NULL          COMMENT '题干快照',
+    `type`             VARCHAR(32)  DEFAULT NULL          COMMENT '题型快照',
+    `difficulty`       VARCHAR(32)  DEFAULT NULL          COMMENT '难度快照',
+    `options`          JSON         DEFAULT NULL          COMMENT '选项快照 JSON（不含 isCorrect）',
+    `content_version`  VARCHAR(64)  NOT NULL DEFAULT ''   COMMENT '题目内容版本',
+    -- availability
+    `available`        BIT(1)       NOT NULL DEFAULT b'1' COMMENT '源资源是否可用',
+    -- audit
+    `creator`          VARCHAR(64)  DEFAULT ''            COMMENT '创建者',
+    `create_time`      DATETIME     NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
+    `updater`          VARCHAR(64)  DEFAULT ''            COMMENT '更新者',
+    `update_time`      DATETIME     NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '更新时间',
+    `deleted`          BIT(1)       NOT NULL DEFAULT b'0' COMMENT '是否删除',
+    PRIMARY KEY (`id`),
+    UNIQUE KEY `uk_tenant_user_target` (`tenant_id`, `user_id`, `target_type`, `target_id`),
+    KEY `idx_tenant_user` (`tenant_id`, `user_id`),
+    KEY `idx_tenant_user_target_type` (`tenant_id`, `user_id`, `target_type`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='教育-收藏夹';
+
+-- =============================================
+-- Post-migration verification queries
+-- =============================================
+-- Verify new table exists:
+--   SHOW CREATE TABLE education_favorite;
+-- Verify unique key is enforced:
+--   SHOW INDEX FROM education_favorite WHERE Key_name = 'uk_tenant_user_target';
+-- Verify no orphan data (should be 0 after fresh migration):
+--   SELECT COUNT(*) FROM education_favorite;