feat(chat): add endpoint to get quoted message by ID

Add GET /api/v1/messages/:id endpoint to retrieve quoted/replied messages
for preview fill and navigation purposes. The endpoint includes authorization
checks ensuring only conversation participants can access the message.

Additionally improve file upload handling for native clients (expo-file-system)
by accepting explicit displayName from frontend, with sanitization to prevent
path traversal and 255-character length limit. File extension detection now
prioritizes the real display name over the multipart header filename.

internal/handler/message_handler.go

@@ -358,6 +358,37 @@ func (h *MessageHandler) GetMessages(c *gin.Context) {
 	response.Paginated(c, result, total, page, pageSize)
 }
 
+// GetReplyMessage 按消息 ID 获取被引用消息（用于引用预览回填/跳转）
+// GET /api/v1/messages/:id
+func (h *MessageHandler) GetReplyMessage(c *gin.Context) {
+	userID := c.GetString("user_id")
+	if userID == "" {
+		response.Unauthorized(c, "")
+		return
+	}
+
+	messageID := c.Param("id")
+	if messageID == "" {
+		response.BadRequest(c, "message id is required")
+		return
+	}
+
+	message, err := h.chatService.GetReplyMessage(c.Request.Context(), messageID, userID)
+	if err != nil {
+		response.BadRequest(c, err.Error())
+		return
+	}
+
+	// 转换为响应格式（标记已过期文件 / 撤回占位等，保持与列表接口一致）
+	result := h.convertMessagesWithExpiry(c.Request.Context(), []*model.Message{message})
+	if len(result) == 0 {
+		response.InternalServerError(c, "failed to convert message")
+		return
+	}
+
+	response.Success(c, result[0])
+}
+
 // SendMessage 发送消息
 // POST /api/conversations/:id/messages
 func (h *MessageHandler) SendMessage(c *gin.Context) {

internal/handler/upload_handler.go

@@ -109,8 +109,12 @@ func (h *UploadHandler) UploadFile(c *gin.Context) {
 	}
 
 	folder := c.DefaultPostForm("folder", "chat")
+	// 前端（尤其原生端 expo-file-system）无法自定义 multipart filename，
+	// 文件常被复制到缓存目录并以 UUID 命名，导致 header Filename 丢失真实名。
+	// 因此前端额外把真实文件名通过 "name" 字段回传，作为权威展示名。
+	displayName := c.PostForm("name")
 
-	result, err := h.uploadService.UploadFile(c.Request.Context(), file, folder, userID)
+	result, err := h.uploadService.UploadFile(c.Request.Context(), file, folder, userID, displayName)
 	if err != nil {
 		// 参数类错误（大小/类型）返回 400，其余返回 500
 		if file.Size > 0 && file.Size <= 100<<20 {