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/challenge.py

@@ -0,0 +1,36 @@
+from datetime import datetime
+from enum import StrEnum
+import uuid
+
+from sqlalchemy import JSON, DateTime, Enum, ForeignKey, String, Text, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.database import Base
+
+
+class ChallengeStatus(StrEnum):
+    pending = "pending"
+    approved = "approved"
+    denied = "denied"
+    expired = "expired"
+
+
+class Challenge(Base):
+    __tablename__ = "challenges"
+    __table_args__ = (UniqueConstraint("nonce", name="uq_challenges_nonce"),)
+
+    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)
+    device_id: Mapped[uuid.UUID | None] = mapped_column(ForeignKey("devices.id", ondelete="SET NULL"), nullable=True, index=True)
+    status: Mapped[ChallengeStatus] = mapped_column(Enum(ChallengeStatus), default=ChallengeStatus.pending, index=True)
+    relying_party: Mapped[str] = mapped_column(String(255))
+    requester_ip: Mapped[str] = mapped_column(String(64))
+    location: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    nonce: Mapped[str] = mapped_column(String(128), index=True)
+    payload: Mapped[dict] = mapped_column(JSON)
+    signature: Mapped[str | None] = mapped_column(Text, nullable=True)
+    issued_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), index=True)
+    expires_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), index=True)
+    responded_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    oidc_state: Mapped[str | None] = mapped_column(String(255), nullable=True, index=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())