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/cmd/server/main.go

@@ -13,12 +13,11 @@ import (
 
 	"github.com/hhs/camtalk/internal/api"
 	"github.com/hhs/camtalk/internal/auth"
-	"github.com/hhs/camtalk/internal/ai/llm"
 	"github.com/hhs/camtalk/internal/ai/stt"
 	"github.com/hhs/camtalk/internal/ai/tts"
 	"github.com/hhs/camtalk/internal/config"
+	eino "github.com/hhs/camtalk/internal/eino"
 	"github.com/hhs/camtalk/internal/logger"
-	"github.com/hhs/camtalk/internal/orchestrator"
 	"github.com/hhs/camtalk/internal/session"
 	"github.com/hhs/camtalk/internal/store"
 	"github.com/hhs/camtalk/internal/ws"
@@ -116,9 +115,6 @@ func main() {
 		sttService = stt.NewDeepgramService(cfg.AI.STT.APIKey, cfg.AI.STT.Model, cfg.AI.STT.Endpoint, cfg.AI.STT.Timeout, logger.Log)
 		logger.Log.Infow("STT service initialized", "provider", "deepgram", "model", cfg.AI.STT.Model)
 	}
-	llmService := llm.NewOpenAIService(cfg.AI.LLM.APIKey, cfg.AI.LLM.Model, cfg.AI.LLM.Endpoint, cfg.AI.LLM.Timeout, cfg.AI.LLM.HTTPClientTimeout, logger.Log)
-	logger.Log.Infow("LLM service initialized", "provider", cfg.AI.LLM.Provider, "model", cfg.AI.LLM.Model, "endpoint", cfg.AI.LLM.Endpoint, "timeout", cfg.AI.LLM.Timeout)
-
 	var ttsService tts.Service
 	switch strings.ToLower(cfg.AI.TTS.Provider) {
 	case "mimo", "xiaomi":
@@ -129,8 +125,12 @@ func main() {
 		logger.Log.Infow("TTS service initialized", "provider", "openai", "model", cfg.AI.TTS.Model, "voice", cfg.AI.TTS.Voice, "speed", cfg.AI.TTS.Speed)
 	}
 
-	// 初始化 Orchestrator
-	orch := orchestrator.New(sttService, llmService, ttsService, sessionMgr, cfg)
+	// 初始化 Eino Graph + Orchestrator
+	pipelineGraph, err := eino.NewPipelineGraph(ctx, cfg, sttService, ttsService, sessionMgr)
+	if err != nil {
+		logger.Log.Fatalw("failed to create eino pipeline graph", "error", err)
+	}
+	orch := eino.NewEinoOrchestrator(pipelineGraph, sessionMgr, cfg.AI.LLM.Model)
 
 	// 初始化认证服务
 	tokenMgr := auth.NewTokenManager(