feat: 新增 MySQL 专项测试能力

- 新增 backend/tests/mysql/ 目录，包含 Docker Compose 配置和测试文件
- 新增 Makefile 命令: test-mysql, test-mysql-up, test-mysql-down, test-mysql-quick
- 使用 build tag 控制测试启用，默认不运行
- 测试覆盖: 迁移正确性、外键约束、UNIQUE 约束、并发写入
- 发现 statsRepo.Record 存在并发 bug（检查-然后-操作竞态条件）

backend/tests/mysql/docker-compose.yml

@@ -0,0 +1,21 @@
+version: '3.8'
+
+services:
+  mysql:
+    image: mysql:8.0
+    container_name: nex-mysql-test
+    environment:
+      MYSQL_ROOT_PASSWORD: testpass
+      MYSQL_DATABASE: nex_test
+      MYSQL_USER: nex_test
+      MYSQL_PASSWORD: testpass
+    ports:
+      - "13306:3306"
+    tmpfs:
+      - /var/lib/mysql
+    healthcheck:
+      test: ["CMD", "mysqladmin", "ping", "-h", "localhost", "-u", "root", "-p$$MYSQL_ROOT_PASSWORD"]
+      interval: 1s
+      timeout: 5s
+      retries: 10
+      start_period: 10s