Implement WebAuthn passkey authentication
Add complete WebAuthn support for passkey registration and authentication:
- Install @simplewebauthn/server for WebAuthn utilities
- Create passkey-helpers.ts with base64url/Uint8Array conversion utilities
- Create webauthn.ts with registration/authentication option generation and verification
- Create context.ts with API context types
- Implement all WebAuthn router handlers (createRegistrationOptions, verifyRegistration, createAuthenticationOptions, verifyAuthentication)
- Implement passkey management handlers (listPasskeys, createPasskey, renamePasskey, deletePasskey)
- Add WebAuthn configuration constants and environment variables
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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