feat: return conversation id

src/dto/api.rs

@@ -155,6 +155,8 @@ pub struct ChatCompletionResponse {
     pub model: String,
     pub choices: Vec<ChatChoice>,
     pub usage: Option<Usage>, // optional (Ollama may not always provide)
+
+    pub conversation_id: Option<Uuid>,
 }
 
 #[derive(Debug, Serialize, Deserialize, ToSchema)]