chore: 后端首次基线提交

运行必须源码：app/(controller/model/service/middleware/lib/utils/validate/router)、config/、database/(migrations/seeders/sql)、启动脚本、app/public/templates 模板。
已忽略运行非必须：node_modules/、logs/、run/、app/public 运行时上传数据、.env*、构建/缓存产物。

database/migrations/021-unique-submission-indexes.js

@@ -0,0 +1,99 @@
+'use strict';
+
+/**
+ * 迁移 021：为答题提交表增加唯一约束，杜绝并发作答重复落库（Bug #12 / B12）
+ *
+ * 背景（B12）：原 saveAnswers 使用「先 findOne 再 create」的非原子写法，且
+ * (aggregation_uid, question_uid, question_type) 仅为普通索引。同一用户并发提交
+ * 时，多个请求都 find 不到记录，于是各自 create，导致同一题被插入多行
+ * （实测：3 路并发产生 57 行 / 20 题，exam_submissions 出现 3 条同选手记录）。
+ * 重复行会污染 answeredCount、判分明细与成绩恢复，属数据一致性缺陷。
+ *
+ * 修复：
+ * 1) zhongzhi_submissions / gaozhi_submissions 的复合索引升级为唯一索引
+ *    uk_{table}_submission (aggregation_uid, question_uid, question_type)；
+ *    service.saveAnswers 改用 upsert(conflictFields=[...])，由 MySQL 的
+ *    INSERT ... ON DUPLICATE KEY UPDATE 在并发下原子折叠为单行更新。
+ * 2) exam_submissions 的 contestant_id 索引升级为唯一索引
+ *    uk_exam_submission_contestant，配合 service 中并发安全的
+ *    _ensureExamSubmissionSafe，保证每选手仅一条提交记录。
+ *
+ * 注意：up/down 均先去重（保留最小 id）再加减索引，确保幂等且不会因历史脏数据失败。
+ */
+module.exports = {
+  up: async (queryInterface) => {
+    // ---- 去重：同 (aggregation_uid, question_uid, question_type) 仅留最小 id ----
+    await queryInterface.sequelize.query(`
+      DELETE t FROM zhongzhi_submissions t
+      WHERE t.id NOT IN (
+        SELECT min_id FROM (
+          SELECT MIN(id) AS min_id FROM zhongzhi_submissions
+          GROUP BY aggregation_uid, question_uid, question_type
+        ) AS keep
+      )
+    `);
+    await queryInterface.sequelize.query(`
+      DELETE t FROM gaozhi_submissions t
+      WHERE t.id NOT IN (
+        SELECT min_id FROM (
+          SELECT MIN(id) AS min_id FROM gaozhi_submissions
+          GROUP BY aggregation_uid, question_uid, question_type
+        ) AS keep
+      )
+    `);
+    // exam_submissions：每 contestant_id 仅留最小 id
+    await queryInterface.sequelize.query(`
+      DELETE t FROM exam_submissions t
+      WHERE t.id NOT IN (
+        SELECT min_id FROM (
+          SELECT MIN(id) AS min_id FROM exam_submissions
+          GROUP BY contestant_id
+        ) AS keep
+      )
+    `);
+
+    // ---- zhongzhi ----
+    await queryInterface.removeIndex('zhongzhi_submissions', 'idx_zhongzhi_submissions_agg_qtype').catch(() => {});
+    await queryInterface.addIndex('zhongzhi_submissions', {
+      name: 'uk_zhongzhi_submission',
+      unique: true,
+      fields: ['aggregation_uid', 'question_uid', 'question_type'],
+    }).catch(() => {});
+
+    // ---- gaozhi ----
+    await queryInterface.removeIndex('gaozhi_submissions', 'idx_gaozhi_submissions_agg_qtype').catch(() => {});
+    await queryInterface.addIndex('gaozhi_submissions', {
+      name: 'uk_gaozhi_submission',
+      unique: true,
+      fields: ['aggregation_uid', 'question_uid', 'question_type'],
+    }).catch(() => {});
+
+    // ---- exam_submissions ----
+    await queryInterface.removeIndex('exam_submissions', 'idx_exam_submissions_contestant_id').catch(() => {});
+    await queryInterface.addIndex('exam_submissions', {
+      name: 'uk_exam_submission_contestant',
+      unique: true,
+      fields: ['contestant_id'],
+    }).catch(() => {});
+  },
+
+  down: async (queryInterface) => {
+    await queryInterface.removeIndex('zhongzhi_submissions', 'uk_zhongzhi_submission').catch(() => {});
+    await queryInterface.addIndex('zhongzhi_submissions', {
+      name: 'idx_zhongzhi_submissions_agg_qtype',
+      fields: ['aggregation_uid', 'question_uid', 'question_type'],
+    });
+
+    await queryInterface.removeIndex('gaozhi_submissions', 'uk_gaozhi_submission').catch(() => {});
+    await queryInterface.addIndex('gaozhi_submissions', {
+      name: 'idx_gaozhi_submissions_agg_qtype',
+      fields: ['aggregation_uid', 'question_uid', 'question_type'],
+    });
+
+    await queryInterface.removeIndex('exam_submissions', 'uk_exam_submission_contestant').catch(() => {});
+    await queryInterface.addIndex('exam_submissions', {
+      name: 'idx_exam_submissions_contestant_id',
+      fields: ['contestant_id'],
+    });
+  },
+};