fix: 修复 Eino Graph 类型不匹配和多模态消息问题

- 添加 msg2str 转换节点解决 ChatModel 输出 *schema.Message 与 Splitter 期望 string 的类型不匹配
- 将多模态图片内容从 system 消息移到 user 消息（DashScope API 仅支持 user/tool 角色的多模态内容）
- 修复 Content 和 UserInputMultiContent 不能同时设置的问题
- Splitter 输出改为 StreamReader[string]（单句），TTS 改为 TransformableLambda 流式消费
- 修复 .env 中 PostgreSQL DSN 和 Redis ADDR 的 http:// 前缀问题

backend/internal/eino/graph.go

@@ -18,8 +18,9 @@ import (
 const (
 	nodeSTT      = "stt"
 	nodeHistory  = "history"
-	nodeLLM      = "llm"
-	nodeSplitter = "splitter"
+	nodeLLM            = "llm"
+	nodeMessageToString = "msg2str"
+	nodeSplitter       = "splitter"
 	nodeTTS      = "tts"
 	nodeDone     = "done"
 )
@@ -69,6 +70,7 @@ func NewPipelineGraph(
 	_ = g.AddLambdaNode(nodeSTT, NewSTTLambda(sttService))
 	_ = g.AddLambdaNode(nodeHistory, NewHistoryLambda(sessionMgr.GetHistory, maxHistory))
 	_ = g.AddChatModelNode(nodeLLM, chatModel)
+	_ = g.AddLambdaNode(nodeMessageToString, NewMessageToStringLambda())
 	_ = g.AddLambdaNode(nodeSplitter, NewSplitterLambda())
 	_ = g.AddLambdaNode(nodeTTS, NewTTSLambda(
 		ttsService,
@@ -83,7 +85,8 @@ func NewPipelineGraph(
 	_ = g.AddEdge(compose.START, nodeSTT)
 	_ = g.AddEdge(nodeSTT, nodeHistory)
 	_ = g.AddEdge(nodeHistory, nodeLLM)
-	_ = g.AddEdge(nodeLLM, nodeSplitter)
+	_ = g.AddEdge(nodeLLM, nodeMessageToString)
+	_ = g.AddEdge(nodeMessageToString, nodeSplitter)
 	_ = g.AddEdge(nodeSplitter, nodeTTS)
 	_ = g.AddEdge(nodeTTS, nodeDone)
 	_ = g.AddEdge(nodeDone, compose.END)