feat: 实现 Eino Graph 构建与 Orchestrator 适配器，切换 main.go

- graph.go: 构建 Graph 拓扑 START→STT→History→ChatModel→Splitter→TTS→Done→END
  - 创建 eino-ext ChatModel 对接 DashScope OpenAI 兼容接口
  - 统一使用值类型（PipelineInput/PipelineOutput）
  - Callback 在运行时通过 Stream option 传入
- adapter.go: EinoOrchestrator 实现 orchestrator.Orchestrator 接口
  - 解码 base64 音频/图片，注入 context 值
  - 调用 Graph.Stream() 触发惰性执行并消费输出
  - 追加用户/助手消息到历史
- main.go: 移除旧 llmService + orchestrator.New()
  替换为 eino.NewPipelineGraph() + eino.NewEinoOrchestrator()
- 各节点统一使用值类型，State 传递请求元数据

Co-Authored-By: Claude <noreply@anthropic.com>

backend/internal/eino/adapter.go

@@ -0,0 +1,165 @@
+package eino
+
+import (
+	"context"
+	"encoding/base64"
+	"io"
+	"time"
+
+	"github.com/cloudwego/eino/compose"
+
+	"github.com/hhs/camtalk/internal/logger"
+	"github.com/hhs/camtalk/internal/models"
+	"github.com/hhs/camtalk/internal/orchestrator"
+	"github.com/hhs/camtalk/internal/session"
+)
+
+// ctxKeySessionID sessionID 的 context key。
+type ctxKeySessionID struct{}
+
+// WithSessionID 将 sessionID 注入 context。
+func WithSessionID(ctx context.Context, sessionID string) context.Context {
+	return context.WithValue(ctx, ctxKeySessionID{}, sessionID)
+}
+
+// EinoOrchestrator 实现 orchestrator.Orchestrator 接口。
+// 将 Eino Graph 包装为现有接口，WS Handler 几乎不用改。
+type EinoOrchestrator struct {
+	graph      *PipelineGraph
+	sessionMgr session.Manager
+	model      string
+	callbacks  compose.Option // 运行时 Callback option
+}
+
+// NewEinoOrchestrator 创建 Eino 编排器适配器。
+func NewEinoOrchestrator(graph *PipelineGraph, sessionMgr session.Manager, model string) *EinoOrchestrator {
+	return &EinoOrchestrator{
+		graph:      graph,
+		sessionMgr: sessionMgr,
+		model:      model,
+		callbacks:  compose.WithCallbacks(BuildCallbackHandler()),
+	}
+}
+
+// ProcessQuery 实现 orchestrator.Orchestrator 接口。
+func (e *EinoOrchestrator) ProcessQuery(
+	ctx context.Context,
+	sessionID string,
+	req models.WsQuery,
+	history []models.Message,
+	sender orchestrator.Sender,
+) error {
+	log := logger.Log
+	startTime := time.Now()
+
+	// 1. 设置活跃请求
+	if err := e.sessionMgr.SetActiveRequest(ctx, sessionID, req.RequestID); err != nil {
+		log.Errorw("设置活跃请求失败", "error", err)
+	}
+	defer e.sessionMgr.ClearActiveRequest(ctx, sessionID)
+
+	// 2. 获取会话配置
+	sess, err := e.sessionMgr.Get(ctx, sessionID)
+	if err != nil {
+		log.Errorw("获取会话失败", "error", err)
+		sender.SendError(models.WsError{
+			Type:      "error",
+			RequestID: req.RequestID,
+			Code:      "SESSION_NOT_FOUND",
+			Message:   "会话不存在",
+		})
+		return err
+	}
+
+	// 3. 解码音频和图片
+	var audioData []byte
+	if req.Text == "" && req.Audio != "" {
+		audioData, err = base64.StdEncoding.DecodeString(req.Audio)
+		if err != nil {
+			log.Errorw("音频解码失败", "error", err)
+			sender.SendError(models.WsError{
+				Type:      "error",
+				RequestID: req.RequestID,
+				Code:      "INVALID_MESSAGE",
+				Message:   "音频数据解码失败",
+			})
+			return err
+		}
+	}
+
+	var imageData []byte
+	if req.Image != "" {
+		imageData, err = base64.StdEncoding.DecodeString(req.Image)
+		if err != nil {
+			log.Errorw("图片解码失败", "error", err)
+			sender.SendError(models.WsError{
+				Type:      "error",
+				RequestID: req.RequestID,
+				Code:      "INVALID_MESSAGE",
+				Message:   "图片数据解码失败",
+			})
+			return err
+		}
+	}
+
+	// 4. 构建 Graph 输入
+	input := buildPipelineInput(req, sessionID, sess, audioData, imageData)
+
+	// 5. 注入 context 值（供 Callback 和 Lambda 节点使用）
+	ctx = WithSender(ctx, sender)
+	ctx = WithRequestID(ctx, req.RequestID)
+	ctx = WithSessionID(ctx, sessionID)
+	ctx = WithStartTime(ctx, startTime)
+	ctx = WithPipelineState(ctx, genLocalState(ctx))
+
+	// 6. 追加用户消息到历史
+	if req.Text != "" {
+		_ = e.sessionMgr.AppendMessage(ctx, sessionID, models.Message{
+			Role:    "user",
+			Content: req.Text,
+		})
+	}
+
+	// 7. 调用 Graph（Stream 模式 + 运行时 Callback）
+	streamReader, err := e.graph.Runnable.Stream(ctx, input, e.callbacks)
+	if err != nil {
+		log.Errorw("Graph Stream 启动失败", "error", err)
+		sender.SendError(models.WsError{
+			Type:      "error",
+			RequestID: req.RequestID,
+			Code:      "INTERNAL_ERROR",
+			Message:   "编排器启动失败",
+		})
+		return err
+	}
+
+	// 8. 消费 StreamReader（触发整条链路执行，side effects 推送消息到客户端）
+	var output PipelineOutput
+	for {
+		o, err := streamReader.Recv()
+		if err != nil {
+			if err == io.EOF {
+				break
+			}
+			log.Errorw("Graph Stream 消费错误", "error", err)
+			break
+		}
+		output = o
+	}
+
+	// 9. 追加助手消息到历史
+	if output.FullResponse != "" {
+		_ = e.sessionMgr.AppendMessage(ctx, sessionID, models.Message{
+			Role:    "assistant",
+			Content: output.FullResponse,
+		})
+	}
+
+	latency := time.Since(startTime).Milliseconds()
+	log.Infow("Eino 编排完成",
+		"request_id", req.RequestID,
+		"latency_ms", latency,
+		"session_id", sessionID)
+
+	return nil
+}