fix: lazy-init OpenAI client to avoid build failure without API key

src/lib/llm/providers/openai.ts

@@ -6,9 +6,14 @@ import type {
   ItemResearch,
 } from "../types";
 
-const client = new OpenAI({
-  apiKey: process.env.OPENAI_API_KEY,
-});
+let _client: OpenAI | null = null;
+
+function getClient(): OpenAI {
+  if (!_client) {
+    _client = new OpenAI({ apiKey: process.env.OPENAI_API_KEY });
+  }
+  return _client;
+}
 
 const SYSTEM_PROMPT = `You are an expert research analyst. Your job is to compare items across multiple dimensions and produce structured, insightful comparison data.
 
@@ -105,7 +110,7 @@ Provide a comprehensive comparison with scores, pros/cons, and a recommendation.
 
   for (let attempt = 1; attempt <= MAX_RETRIES; attempt++) {
     try {
-      const response = await client.chat.completions.create({
+      const response = await getClient().chat.completions.create({
         model: "gpt-4o-mini",
         messages: [
           { role: "system", content: SYSTEM_PROMPT },