chore: initial project setup with backend, frontend, Android app, and CI/CD

Add complete NexaMFA push MFA system with:
- FastAPI backend with PostgreSQL, Redis, OIDC provider, and Prometheus metrics
- React TypeScript admin console
- Android Kotlin/Jetpack Compose app with biometric authentication
- Docker Compose deployment configuration
- Gitea CI workflow for backend, frontend, and Android builds
- Environment configuration template with security settings
- Documentation for security model, deployment

backend/app/models/__init__.py

@@ -0,0 +1,8 @@
+from app.models.audit import AuditLog
+from app.models.challenge import Challenge, ChallengeStatus
+from app.models.device import Device
+from app.models.enrollment import Enrollment
+from app.models.oidc import AuthorizationCode
+from app.models.user import User
+
+__all__ = ["AuditLog", "AuthorizationCode", "Challenge", "ChallengeStatus", "Device", "Enrollment", "User"]