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/go.mod

@@ -3,6 +3,8 @@ module github.com/hhs/camtalk
 go 1.25.0
 
 require (
+	github.com/cloudwego/eino v0.9.9
+	github.com/cloudwego/eino-ext/components/model/openai v0.1.13
 	github.com/gin-gonic/gin v1.10.0
 	github.com/golang-jwt/jwt/v5 v5.3.1
 	github.com/google/uuid v1.6.0
@@ -24,8 +26,6 @@ require (
 	github.com/bytedance/sonic/loader v0.5.0 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
-	github.com/cloudwego/eino v0.9.9 // indirect
-	github.com/cloudwego/eino-ext/components/model/openai v0.1.13 // indirect
 	github.com/cloudwego/eino-ext/libs/acl/openai v0.1.17 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect