fix: use correct env var OPENAI_BASE_URL for camel-ai LLM routing

camel-ai's OpenAI model reads OPENAI_BASE_URL, not OPENAI_API_BASE_URL.
This caused all simulation LLM calls to go to api.openai.com instead of
the configured provider (DeepSeek, Xiaomi Mimo, etc), resulting in 401.

backend/scripts/run_reddit_simulation.py

@@ -457,7 +457,7 @@ class RedditSimulationRunner:
             raise ValueError("缺少 API Key 配置，请在项目根目录 .env 文件中设置 LLM_API_KEY")
         
         if llm_base_url:
-            os.environ["OPENAI_API_BASE_URL"] = llm_base_url
+            os.environ["OPENAI_BASE_URL"] = llm_base_url
         
         print(f"LLM配置: model={llm_model}, base_url={llm_base_url[:40] if llm_base_url else '默认'}...")