refactor: 使用 openai SDK 替换 fetch 调用 LLM

- 安装 openai 包
- 重命名 zhipu.ts -> llm.ts
- 使用 OpenAI SDK 替代原生 fetch 实现
- 更新所有导入路径

src/lib/bigmodel/llm.ts

@@ -0,0 +1,37 @@
+"use server";
+
+import OpenAI from "openai";
+
+const openai = new OpenAI({
+  apiKey: process.env.ZHIPU_API_KEY,
+  baseURL: "https://open.bigmodel.cn/api/paas/v4",
+});
+
+type Messages = Array<
+  | { role: "system"; content: string }
+  | { role: "user"; content: string }
+  | { role: "assistant"; content: string }
+>;
+
+async function getAnswer(prompt: string): Promise<string>;
+async function getAnswer(prompt: Messages): Promise<string>;
+async function getAnswer(prompt: string | Messages): Promise<string> {
+  const messages: Messages = typeof prompt === "string"
+    ? [{ role: "user", content: prompt }]
+    : prompt;
+
+  const response = await openai.chat.completions.create({
+    model: process.env.ZHIPU_MODEL_NAME || "glm-4",
+    messages: messages as OpenAI.Chat.Completions.ChatCompletionMessageParam[],
+    temperature: 0.2,
+  });
+
+  const content = response.choices[0]?.message?.content;
+  if (!content) {
+    throw new Error("AI API 返回空响应");
+  }
+
+  return content.trim();
+}
+
+export { getAnswer };