build: e2e 测试添加 build tag 避免 CI 运行

- 添加 //go:build e2e tag，CI 不会自动运行这些测试
- Makefile 添加 test-e2e 目标用于本地手动运行

backend/Makefile

-.PHONY: wire build build-embed test-unit test-integration test-cover-integration clean-coverage
+.PHONY: wire build build-embed test-unit test-integration test-e2e test-cover-integration clean-coverage
 
 wire:
 	@echo "生成 Wire 代码..."
@@ -21,6 +21,10 @@ test-unit:
 test-integration:
 	@go test -tags integration ./... -count=1 -race -parallel=8
 
+test-e2e:
+	@echo "运行 E2E 测试（需要本地服务器运行）..."
+	@go test -tags e2e ./internal/integration/... -count=1 -v
+
 test-cover-integration:
 	@echo "运行集成测试并生成覆盖率报告..."
 	@go test -tags=integration -cover -coverprofile=coverage.out -count=1 -race -parallel=8 ./...

backend/internal/integration/e2e_gateway_test.go

+//go:build e2e
+
 package integration
 
 import (