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/core/database.py

@@ -0,0 +1,32 @@
+from collections.abc import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+from sqlalchemy.orm import DeclarativeBase
+
+from app.core.config import get_settings
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+settings = get_settings()
+engine = create_async_engine(settings.database_url, pool_pre_ping=True)
+SessionLocal = async_sessionmaker(engine, expire_on_commit=False, class_=AsyncSession)
+
+
+async def get_session() -> AsyncGenerator[AsyncSession, None]:
+    async with SessionLocal() as session:
+        yield session
+
+
+async def create_all() -> None:
+    import app.models.audit  # noqa: F401
+    import app.models.challenge  # noqa: F401
+    import app.models.device  # noqa: F401
+    import app.models.enrollment  # noqa: F401
+    import app.models.oidc  # noqa: F401
+    import app.models.user  # noqa: F401
+
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)