Protect API endpoints with authentication

[coderabbitai]

Description

Add authentication mechanism to protect API endpoints from unauthorized access.

Requirements

• Implement API key-based authentication and/or OAuth
• Secure endpoints: /api/scan, /api/fix, /api/webhook
• Support multiple authentication methods (API keys, OAuth)
• Add middleware for authentication checks
• Document authentication flow in API docs

Context

Requested by @haroon0x in issue #1

This is critical for production deployment to prevent unauthorized use of the scanning service.