Add AgentSessionService and refactor agent routes

Move session-related responsibilities into a new application-layer AgentSessionService (and AgentSessionFeedbackResult dataclass), provide a bootstrap factory (get_agent_session_service), and update agent API routes to call the service instead of accessing ConversationStore directly. Routes now translate ValueError into 404 responses and use service methods for get/list/history/delete/feedback. Also update package exports and docs/READMEs to declare the backend architecture authority, enforce api -> application -> domain ports -> infrastructure boundaries, and call out legacy services/workflows as migration-only. These changes centralize session logic in the application layer and tighten architecture guidance for future backend work.

README.md

@@ -22,7 +22,10 @@ AIRegulation-DocAnalysis-Demo/
 │   │   ├── application/        # 用例编排层
 │   │   ├── domain/             # 领域模型与稳定端口
 │   │   ├── infrastructure/     # MinIO / Milvus / 阿里云 / embedding / session 适配
+│   │   ├── shared/             # 组合根、配置无关 wiring 与横切支撑
 │   │   ├── config/             # 配置与日志
+│   │   ├── services/           # 迁移期 legacy façade，不是新增业务逻辑默认落点
+│   │   ├── workflows/          # 迁移期 legacy workflow，不是新增业务逻辑默认落点
 │   │   └── workers/
 │   ├── requirements.txt
 │   └── main.py
@@ -64,6 +67,13 @@ docker-compose logs -f milvus
 
 ## API接口
 
+## Backend Architecture
+
+- Backend 架构规范文档：`docs/architecture/backend-project-architecture.md`
+- Backend 迁移 RFC：`docs/rfc/backend-api-parsing-embedding-migration-requirements.md`
+- 后续 backend 新增功能、重构和技术替换必须同时满足 RFC 与架构文档。
+- `backend/app/services/*` 与 `backend/app/workflows/*` 当前属于迁移期遗留目录，除迁移或兼容修复外，不应继续承载新的业务编排。
+
 ### 上传文档
 
 ```bash