refactor: fix linting issues and code quality

- Fix import ordering in __init__.py
- Remove unused imports from dependencies.py
- Fix import sorting across multiple files
- Apply ruff auto-fixes

No functional changes

src/notebooklm_agent/core/config.py

@@ -66,7 +66,7 @@ class Settings(BaseSettings):
         return not self.debug and not self.testing
 
 
-@lru_cache()
+@lru_cache
 def get_settings() -> Settings:
     """Get cached settings instance.