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/enrollment.py

@@ -0,0 +1,18 @@
+from datetime import datetime
+import uuid
+
+from sqlalchemy import Boolean, DateTime, ForeignKey, String, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.database import Base
+
+
+class Enrollment(Base):
+    __tablename__ = "enrollments"
+
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+    user_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), index=True)
+    token_hash: Mapped[str] = mapped_column(String(128), unique=True, index=True)
+    expires_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), index=True)
+    used: Mapped[bool] = mapped_column(Boolean, default=False)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())