Add RAG configuration options and enhance parameter configurability

- Add top-k and cosine-threshold parms for api server
- Update .env and cli parms handling with new parameters
- Improve splash screen display
- Update bash and storage classes to read new parameters from .env file.

lightrag/kg/nano_vector_db_impl.py

@@ -73,7 +73,7 @@ from lightrag.base import (
 
 @dataclass
 class NanoVectorDBStorage(BaseVectorStorage):
-    cosine_better_than_threshold: float = 0.2
+    cosine_better_than_threshold: float = float(os.getenv("COSINE_THRESHOLD", "0.2"))
 
     def __post_init__(self):
         self._client_file_name = os.path.join(