feat: T03+T04 backend - FastAPI, DB models, Rentman integration, admin auth, APScheduler

backend/app/models/admin_user.py

@@ -0,0 +1,12 @@
+"""Admin user model."""
+from sqlalchemy import Column, Integer, String, TIMESTAMP, func
+from app.database import Base
+
+
+class AdminUser(Base):
+    __tablename__ = "admin_users"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    username = Column(String(64), unique=True, nullable=False)
+    password_hash = Column(String(255), nullable=False)
+    created_at = Column(TIMESTAMP, server_default=func.now())