feat: Add OIDC provider, passkeys, MFA, and admin UI
Major feature release transforming ATAuth into a full SSO provider:
## OIDC Provider
- OpenID Connect discovery endpoints (/.well-known/*)
- Authorization, token, userinfo, revocation endpoints
- ES256/RS256 JWT signing with key rotation
- PKCE support for public clients
## Authentication Methods
- Passkey/WebAuthn support for passwordless login
- TOTP MFA with backup codes
- Email verification and account recovery
## Admin UI
- React + TailwindCSS admin dashboard
- Manage legacy apps and OIDC clients
- Session management and user administration
- Signing key rotation
## Deployment
- Multi-stage Docker build with admin UI
- Kubernetes manifests with staging/production overlays
- Helm chart for flexible deployment
- Gitea Actions and Tekton CI/CD pipelines
- Database backup CronJob
- Safe deployment runbook
## Backward Compatibility
- Legacy HMAC token flow unchanged
- Existing /auth/* routes preserved
- New features are additive (new tables only)