Add Optional API Key Authentication to FastAPI Services

[ParisNeo]

This PR implements optional API key authentication across all FastAPI services in LightRAG. The implementation provides flexible security that can be enabled or disabled through configuration.

Key Changes:

• Added optional API key authentication system across all FastAPI endpoints
• Authentication can be configured through environment variable LIGHTRAG_API_KEY or command line argument
• Services run without authentication when no API key is configured
• Implemented proper security headers and API key validation
• Added OpenAPI documentation support for API key authentication

Implementation Details:

• Used FastAPI's built-in security utilities for robust authentication
• Added X-API-Key header support for API authentication
• Implemented graceful fallback to no-auth when API key is not configured
• Maintained CORS middleware compatibility
• Added proper error handling for authentication failures

Configuration:

• Set API key through environment variable: LIGHTRAG_API_KEY=your_key
• Or through command line: --key your_key
• Leave unset to disable authentication

Testing:

• Tested with authentication enabled/disabled
• Verified CORS functionality
• Confirmed OpenAPI documentation accuracy
• Tested error handling for invalid API keys

This enhancement improves the security posture of LightRAG while maintaining flexibility for different deployment scenarios.