feat: implement WebAuthn credential registration, login verification, and management

Complete the WebAuthn/Passkey flow that was previously stubbed out:
- Add ChallengeStore (Redis + TTLCache fallback) for challenge lifecycle
- Add RP config helper to extract rp_id/origin from site settings
- Fix registration start (exclude_credentials, user_id, challenge storage)
- Implement registration finish (verify + create UserAuthn & AuthIdentity)
- Add authentication options endpoint for Discoverable Credentials login
- Fix passkey login to use challenge_token and base64url encoding
- Add credential management endpoints (list/rename/delete)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sqlmodels/__init__.py

@@ -30,7 +30,12 @@ from .user import (
     UserCalibrateResponse,
     UserAdminDetailResponse,
 )
-from .user_authn import AuthnResponse, UserAuthn
+from .user_authn import (
+    AuthnDetailResponse,
+    AuthnFinishRequest,
+    AuthnRenameRequest,
+    UserAuthn,
+)
 from .color import ChromaticColor, NeutralColor, ThemeColorsBase, BUILTIN_DEFAULT_COLORS
 from .theme_preset import (
     ThemePreset, ThemePresetBase,