Add Passkey Authentication backend with FIDO2/WebAuthn

[gwho]

Implemented a complete passwordless authentication system using FastAPI
and the WebAuthn protocol. This production-ready prototype demonstrates
modern security best practices for passkey-based authentication.

Features:

• Clean separation of Registration and Authentication ceremonies
• Pydantic V2 models for User and Credential entities
• Cryptographic challenge/response flow with detailed security docs
• In-memory database (easily replaceable with PostgreSQL/MongoDB)
• Comprehensive inline documentation explaining crypto operations
• Minimal HTML/JS frontend using navigator.credentials API
• Clone detection via signature counters
• User verification enforcement (biometric/PIN)

Security Architecture:

• Asymmetric cryptography (public/private key pairs)
• One-time challenges prevent replay attacks
• Origin validation prevents phishing
• No passwords or shared secrets transmitted
• Signature verification using stored public keys

Files added:

• main.py: Complete FastAPI backend with WebAuthn endpoints
• requirements.txt: Python dependencies
• README.md: Comprehensive setup and security documentation
• .gitignore: Standard Python/project ignores