feat: Add comprehensive documentation for integration layer, API design, config governance, observability, build/deploy, and testing strategies

- Introduced integration layer design with Resilience4j for external vendor calls.
- Established API design standards with unified response structures and global exception handling.
- Defined configuration and service governance using Kubernetes native solutions.
- Implemented observability practices including trace ID propagation and structured logging.
- Outlined build and multi-environment deployment strategies using Gradle and GitLab CI/CD.
- Specified testing strategies across different layers, utilizing JUnit, MockK, Testcontainers, and WireMock.

README.md

@@ -36,9 +36,25 @@ Flutter APP 的分包、分层、技术选型等决策记录，按序号阅读
 
 `.drawio` 文件可用 [draw.io 桌面版](https://github.com/jgraph/drawio-desktop) 或 VS Code 的 Draw.io Integration 插件打开查看。
 
+### backend/
+
+后端（Kotlin + Spring Boot + Gradle）架构决策文档，按序号阅读：
+
+| 文档 | 内容 |
+| --- | --- |
+| [backend/01-project-structure.md](./backend/01-project-structure.md) | 工程结构 / 模块划分（Gradle 多模块，模块化单体） |
+| [backend/02-layering.md](./backend/02-layering.md) | 分层规范（api/application/domain/infrastructure） |
+| [backend/03-persistence.md](./backend/03-persistence.md) | 持久层方案（Spring Data JPA + Hibernate，Flyway） |
+| [backend/04-security-auth.md](./backend/04-security-auth.md) | 安全与认证方案（Spring Security + JWT，门店上下文） |
+| [backend/05-integration-layer.md](./backend/05-integration-layer.md) | 集成层设计（F6 Adapter / Mini 域客户端） |
+| [backend/06-api-design.md](./backend/06-api-design.md) | API 设计规范（统一响应、DTO、版本化） |
+| [backend/07-config-governance.md](./backend/07-config-governance.md) | 配置与服务治理（K8s ConfigMap/Secret） |
+| [backend/08-observability.md](./backend/08-observability.md) | 可观测性（Trace ID、日志、审计） |
+| [backend/09-build-deploy.md](./backend/09-build-deploy.md) | 构建与多环境部署（Gradle、Docker、GitLab CI/CD） |
+| [backend/10-testing.md](./backend/10-testing.md) | 测试策略 |
+
 ## 待补充
 
-- Backend 设计文档
 - API 文档
 
 ## 语言约定