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/application/agent/services.py

@@ -1,7 +1,8 @@
-"""Implement application-layer logic for services."""
+"""Implement application-layer logic for agent services."""
 
 from __future__ import annotations
 
+from dataclasses import dataclass
 from typing import Generator
 
 from app.domain.conversation import AnswerGenerator, AnswerResult, ConversationStore
@@ -141,5 +142,50 @@ class AgentConversationService:
                 content=full_answer,
                 sources=sources_payload,
             )
-
+        
         return session.session_id, event_stream()
+
+
+@dataclass
+class AgentSessionFeedbackResult:
+    """Represent the result of storing session feedback."""
+
+    session_id: str
+    message_index: int
+
+
+class AgentSessionService:
+    """Provide application-layer access to session management workflows."""
+
+    def __init__(self, *, conversation_store: ConversationStore) -> None:
+        """Initialize the Agent Session Service instance."""
+        self.conversation_store = conversation_store
+
+    def get_session(self, session_id: str):
+        """Return a session by id or raise when it does not exist."""
+        session = self.conversation_store.get_session(session_id)
+        if not session:
+            raise ValueError("会话不存在或已过期")
+        return session
+
+    def get_history(self, *, session_id: str, max_turns: int = 5) -> list[dict[str, str]]:
+        """Return the recent conversation history for a session."""
+        session = self.get_session(session_id)
+        return [{"role": msg.role, "content": msg.content} for msg in session.messages[-(max_turns * 2):]]
+
+    def delete_session(self, session_id: str) -> None:
+        """Delete a session or raise when it does not exist."""
+        if not self.conversation_store.delete_session(session_id):
+            raise ValueError("会话不存在")
+
+    def list_sessions(self) -> list[dict]:
+        """Return the list of visible sessions."""
+        return self.conversation_store.list_sessions()
+
+    def submit_feedback(self, *, session_id: str, message_index: int) -> AgentSessionFeedbackResult:
+        """Validate feedback targets and return a normalized feedback result."""
+        session = self.get_session(session_id)
+        if message_index < 0 or message_index >= len(session.messages):
+            raise ValueError("消息索引不存在")
+        # Preserve the existing API behavior until a persistent feedback store is introduced.
+        return AgentSessionFeedbackResult(session_id=session_id, message_index=message_index)