feat(router): 集成NVIDIA多头分类器实现3-tier智能路由

- 新增nvidia_router.py: 手动加载NVIDIA prompt-task-and-complexity-classifier模型
- DeBERTa-v3-base backbone + 8个分类头(task_type/creativity/reasoning/domain等)
- 综合多维度评分实现simple/medium/complex三级路由
- 映射: simple->qwen-flash, medium->qwen-plus, complex->qwen-max
- main.py切换到NVIDIA路由替代RouteLLM BERT二分类
- 移除LiteLLM依赖解决版本冲突,使用原生httpx调用
- 版本升级至v0.3.0

requirements.txt

@@ -7,5 +7,7 @@ httpx>=0.25.0
 python-dotenv>=1.0.0
 transformers>=4.30.0
 torch>=2.0.0
+# NVIDIA Multi-head Classifier for 3-tier routing
+# nvidia/prompt-task-and-complexity-classifier will be loaded via transformers
 pytest>=7.4.0
 pytest-asyncio>=0.21.0