Make simulation LLM concurrency configurable, default to 5

The hardcoded semaphore=30 caused Groq free tier to hang from
too many concurrent requests. Now reads from config.concurrency
with a safe default of 5.

backend/scripts/run_reddit_simulation.py

@@ -578,7 +578,7 @@ class RedditSimulationRunner:
             agent_graph=self.agent_graph,
             platform=oasis.DefaultPlatformType.REDDIT,
             database_path=db_path,
-            semaphore=30,  # 限制最大并发 LLM 请求数，防止 API 过载
+            semaphore=self.config.get("concurrency", 5),  # 并发LLM请求数，可在配置中调整
         )
         
         await self.env.reset()