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）

Makefile

@@ -2,6 +2,7 @@
 	backend-build backend-run backend-test backend-test-unit backend-test-integration backend-test-coverage \
 	backend-lint backend-deps backend-generate \
 	backend-migrate-up backend-migrate-down backend-migrate-status backend-migrate-create \
+	test-mysql-up test-mysql-down test-mysql test-mysql-quick \
 	frontend-build frontend-dev frontend-test frontend-test-watch frontend-test-coverage frontend-test-e2e frontend-lint \
 	desktop desktop-darwin desktop-windows desktop-linux package-macos
 
@@ -57,6 +58,38 @@ backend-migrate-create:
 	cd backend && goose -dir migrations/sqlite create $$name sql; \
 	cd backend && goose -dir migrations/mysql create $$name sql
 
+# ============================================
+# MySQL 专项测试
+# ============================================
+
+test-mysql-up:
+	@echo "Starting MySQL test container..."
+	cd backend/tests/mysql && docker-compose up -d
+	@echo "Waiting for MySQL to be ready..."
+	@for i in $$(seq 1 30); do \
+		if docker exec nex-mysql-test mysqladmin ping -h localhost -u root -ptestpass --silent 2>/dev/null; then \
+			echo "MySQL is ready!"; \
+			exit 0; \
+		fi; \
+		echo "Waiting... ($$i/30)"; \
+		sleep 1; \
+	done; \
+	echo "MySQL failed to start"; \
+	exit 1
+
+test-mysql-down:
+	@echo "Stopping MySQL test container..."
+	cd backend/tests/mysql && docker-compose down -v
+
+test-mysql: test-mysql-up
+	@echo "Running MySQL tests..."
+	cd backend && go test -tags=mysql ./tests/mysql/... -v -count=1
+	$(MAKE) test-mysql-down
+
+test-mysql-quick:
+	@echo "Running MySQL tests (without container management)..."
+	cd backend && go test -tags=mysql ./tests/mysql/... -v -count=1
+
 # ============================================
 # 前端
 # ============================================