refactor: 重命名文件更清晰，调整最大推理步数为10

- subgraph_builder.py → main_graph_builder.py
- service.py → agent_service.py
- 调整 max_steps 从 40 → 10
- 更新所有相关导入

backend/app/main_graph/utils/visualize_graph.py

@@ -23,7 +23,7 @@ if str(PROJECT_ROOT) not in sys.path:
 
 load_dotenv(PROJECT_ROOT / ".env")
 
-from app.agent.service import AIAgentService
+from app.agent.agent_service import AIAgentService
 from app.config import DB_URI
 from app.main_graph.checkpoint.postgres.aio import AsyncPostgresSaver
 import asyncio