feat(api): 对话路由统一响应模型，新增删除接口；列表负载优化

- API：GET /conversations 使用 List[ConversationResponse]（含 id/title/最近消息）
- API：GET /conversations/{id} 返回对象含 id/title 与完整消息
- API：DELETE /conversations/{id} 新增删除路由，返回 DeleteResult
- API：POST /conversations/{id}/messages 保持 MessageSendResult，敏感词为结构化列表

关联问题：#7 #8 #9 #10 #12

app/api/v1/conversation.py

 from fastapi import APIRouter, Depends, HTTPException, status
 from typing import List
 from app.api.deps import get_current_active_user, require_edition_for_mode
-from app.schemas.conversation import MessageCreate, ConversationDocOut, CreatedId, MessageSendResult
-from app.services.conversation import create_conversation, get_conversation, add_message, get_user_conversations
+from app.schemas.conversation import (
+    MessageCreate,
+    ConversationDocOut,
+    ConversationResponse,
+    CreatedId,
+    MessageSendResult,
+    DeleteResult,
+)
+from app.services.conversation import (
+    create_conversation,
+    get_conversation,
+    add_message,
+    get_user_conversations,
+    delete_conversation,
+)
 
 # 在路由层挂载版别运行模式依赖，限制仅允许当前模式的用户访问
 router = APIRouter(dependencies=[Depends(require_edition_for_mode())])
 
 @router.post("/", status_code=status.HTTP_201_CREATED, response_model=CreatedId)
 async def create_new_conversation(current_user: dict = Depends(get_current_active_user)):
-    """创建新对话"""
+    """创建新对话（返回新建 ID）
+    用途：为当前用户新建会话，并返回新会话的 ID。
+    依赖：鉴权用户、版别运行模式。
+    """
     conversation_id = await create_conversation(str(current_user["_id"]))
     return {"id": conversation_id}
 
-@router.get("/", response_model=List[ConversationDocOut])
+@router.get("/", response_model=List[ConversationResponse])
 async def list_conversations(current_user: dict = Depends(get_current_active_user)):
-    """获取用户的所有对话"""
+    """获取用户的所有对话（列表优化）
+    用途：返回当前用户的对话列表，统一字段并降低负载（仅最近一条消息）。
+    """
     conversations = await get_user_conversations(str(current_user["_id"]))
     return conversations
 
@@ -24,7 +42,9 @@ async def get_single_conversation(
     conversation_id: str,
     current_user: dict = Depends(get_current_active_user)
 ):
-    """获取单个对话"""
+    """获取单个对话详情
+    用途：返回指定对话的完整消息列表，统一 ID 与标题字段。
+    """
     conversation = await get_conversation(conversation_id, str(current_user["_id"]))
     if not conversation:
         raise HTTPException(
@@ -33,13 +53,29 @@ async def get_single_conversation(
         )
     return conversation
 
+@router.delete("/{conversation_id}", response_model=DeleteResult)
+async def remove_conversation(
+    conversation_id: str,
+    current_user: dict = Depends(get_current_active_user)
+):
+    """删除对话
+    用途：仅允许删除当前用户归属的对话，并清理关联敏感记录。
+    返回：删除结果（deleted: bool）
+    """
+    ok = await delete_conversation(conversation_id, str(current_user["_id"]))
+    if not ok:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="对话不存在或无权限")
+    return {"deleted": True, "message": "删除成功"}
+
 @router.post("/{conversation_id}/messages", response_model=MessageSendResult)
 async def send_message(
     conversation_id: str,
     message: MessageCreate,
     current_user: dict = Depends(get_current_active_user)
 ):
-    """发送消息并获取回复"""
+    """发送消息并获取回复
+    用途：在指定对话中发送用户消息；若命中敏感词返回拒绝回复并记录；否则返回模型生成的助手回复。
+    """
     # 检查对话是否存在
     conversation = await get_conversation(conversation_id, str(current_user["_id"]))
     if not conversation: