feat(api): add cursor-based pagination for system messages

- Add SystemMessageCursorPageResponse DTO for cursor pagination
- Implement GetSystemMessagesByCursor handler method
- Add GetByReceiverIDCursor repository method with cursor decoding
- Maintain backward compatibility with existing offset-based pagination

internal/dto/dto.go

@@ -528,6 +528,14 @@ type SystemUnreadCountResponse struct {
 	UnreadCount int64 `json:"unread_count"`
 }
 
+// SystemMessageCursorPageResponse 系统消息游标分页响应
+type SystemMessageCursorPageResponse struct {
+	Items      []*SystemMessageResponse `json:"items"`
+	NextCursor string                   `json:"next_cursor"`
+	PrevCursor string                   `json:"prev_cursor"`
+	HasMore    bool                     `json:"has_more"`
+}
+
 // ==================== 时间格式化 ====================
 
 // FormatTime 格式化时间