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.

backend/app/api/routes/agent.py

@@ -65,7 +65,7 @@ async def chat_with_session(request: ChatRequest):
             model=request.model or settings.llm_model,
             top_k=request.top_k or settings.rag_top_k,
         )
-        session = get_conversation_store().get_session(session_id)
+        session = get_agent_session_service().get_session(session_id)
         return ChatResponse(
             session_id=session_id,
             answer=result.answer,
@@ -133,45 +133,52 @@ async def chat_stream(request: ChatRequest):
 @router.get("/session/{session_id}", response_model=SessionInfo)
 async def get_session_info(session_id: str):
     """Return session info."""
-    session = get_conversation_store().get_session(session_id)
-    if not session:
-        raise HTTPException(status_code=404, detail="会话不存在或已过期")
-    return SessionInfo(
-        session_id=session.session_id,
-        message_count=len(session.messages),
-        created_at=session.created_at,
-        updated_at=session.updated_at,
-    )
+    try:
+        session = get_agent_session_service().get_session(session_id)
+        return SessionInfo(
+            session_id=session.session_id,
+            message_count=len(session.messages),
+            created_at=session.created_at,
+            updated_at=session.updated_at,
+        )
+    except ValueError as exc:
+        raise HTTPException(status_code=404, detail=str(exc))
 
 
 @router.get("/session/{session_id}/history")
 async def get_session_history(session_id: str, max_turns: int = 5):
     """Return session history."""
-    session = get_conversation_store().get_session(session_id)
-    if not session:
-        raise HTTPException(status_code=404, detail="会话不存在或已过期")
-    history = [{"role": msg.role, "content": msg.content} for msg in session.messages[-(max_turns * 2):]]
-    return {"session_id": session_id, "history": history}
+    try:
+        history = get_agent_session_service().get_history(session_id=session_id, max_turns=max_turns)
+        return {"session_id": session_id, "history": history}
+    except ValueError as exc:
+        raise HTTPException(status_code=404, detail=str(exc))
 
 
 @router.delete("/session/{session_id}")
 async def delete_session(session_id: str):
     """Delete session."""
-    if not get_conversation_store().delete_session(session_id):
-        raise HTTPException(status_code=404, detail="会话不存在")
-    return {"message": "会话已删除", "session_id": session_id}
+    try:
+        get_agent_session_service().delete_session(session_id)
+        return {"message": "会话已删除", "session_id": session_id}
+    except ValueError as exc:
+        raise HTTPException(status_code=404, detail=str(exc))
 
 
 @router.get("/sessions", response_model=List[SessionInfo])
 async def list_sessions():
     """List sessions."""
-    return [SessionInfo(**item) for item in get_conversation_store().list_sessions()]
+    return [SessionInfo(**item) for item in get_agent_session_service().list_sessions()]
 
 
 @router.post("/feedback")
 async def submit_feedback(request: FeedbackRequest):
     """Submit feedback."""
-    session = get_conversation_store().get_session(request.session_id)
-    if not session:
-        raise HTTPException(status_code=404, detail="会话不存在")
-    return {"message": "反馈已提交", "session_id": request.session_id, "message_index": request.message_index}
+    try:
+        result = get_agent_session_service().submit_feedback(
+            session_id=request.session_id,
+            message_index=request.message_index,
+        )
+        return {"message": "反馈已提交", "session_id": result.session_id, "message_index": result.message_index}
+    except ValueError as exc:
+        raise HTTPException(status_code=404, detail=str(exc))