refactor: 重构目录结构，后端代码统一到 backend/ 目录

backend/internal/model/chat.go

@@ -0,0 +1,50 @@
+package model
+
+// ChatRole 消息角色
+type ChatRole string
+
+const (
+	ChatRoleSystem    ChatRole = "system"
+	ChatRoleUser      ChatRole = "user"
+	ChatRoleAssistant ChatRole = "assistant"
+	ChatRoleTool      ChatRole = "tool"
+)
+
+// ChatMessage 聊天消息
+type ChatMessage struct {
+	Role       ChatRole
+	Content    string
+	ToolCallID string
+	Name       string
+	ToolCalls  []ChatToolCall
+}
+
+// ChatToolCall 工具调用请求
+type ChatToolCall struct {
+	ID        string
+	Name      string
+	Arguments string
+}
+
+// ChatReply 聊天回复
+type ChatReply struct {
+	Content   string
+	ToolCalls []ChatToolCall
+}
+
+// ChatStreamEvent 流式事件
+type ChatStreamEvent struct {
+	Delta     string
+	ToolCalls []ChatToolCall
+	Done      bool
+}
+
+// ChatContent 聊天输入内容
+type ChatContent struct {
+	Texts []TextPart
+}
+
+// TextPart 文本片段
+type TextPart struct {
+	Message string
+}