Merge branch 'dev-mysql-support' into master

- 新增 MySQL 数据库驱动支持，支持跨设备数据同步
- 新增 MySQL 专项测试能力（并发、约束、迁移）
- 重构迁移目录结构：migrations/sqlite 和 migrations/mysql
- 修复 statsRepo 并发竞态条件，使用 upsert 保证原子性
- Makefile 合并：保留完整命令体系 + 新增 MySQL 测试命令

Makefile

@@ -2,6 +2,7 @@
 	backend-build backend-run backend-dev backend-test backend-test-unit backend-test-integration backend-test-coverage \
 	backend-lint backend-clean backend-deps backend-generate \
 	backend-db-up backend-db-down backend-db-status backend-db-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 frontend-clean \
 	desktop-build desktop-build-mac desktop-build-win desktop-build-linux \
 	desktop-dev desktop-package-mac desktop-package-win desktop-package-linux desktop-clean \
@@ -66,17 +67,53 @@ backend-generate:
 	cd backend && go generate ./...
 
 backend-db-up:
-	cd backend && goose -dir migrations sqlite3 $(DB_PATH) up
+	@echo "Running database migration up..."
+	cd backend && goose -dir migrations/sqlite3 sqlite3 "$(DB_PATH)" up
 
 backend-db-down:
-	cd backend && goose -dir migrations sqlite3 $(DB_PATH) down
+	@echo "Running database migration down..."
+	cd backend && goose -dir migrations/sqlite3 sqlite3 "$(DB_PATH)" down
 
 backend-db-status:
-	cd backend && goose -dir migrations sqlite3 $(DB_PATH) status
+	@echo "Checking database migration status..."
+	cd backend && goose -dir migrations/sqlite3 sqlite3 "$(DB_PATH)" status
 
 backend-db-create:
 	@read -p "Migration name: " name; \
-	cd backend && goose -dir migrations create $$name sql
+	cd backend && goose -dir migrations/sqlite create $$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