Complete backend infrastructure and authentication system

Co-authored-by: mblanke <9078342+mblanke@users.noreply.github.com>

backend/app/models/user.py

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+from sqlalchemy import Column, Integer, String, DateTime, ForeignKey, Boolean
+from sqlalchemy.orm import relationship
+from datetime import datetime
+
+from app.core.database import Base
+
+
+class User(Base):
+    __tablename__ = "users"
+
+    id = Column(Integer, primary_key=True, index=True)
+    username = Column(String, unique=True, index=True, nullable=False)
+    password_hash = Column(String, nullable=False)
+    role = Column(String, default="user", nullable=False)  # user, admin
+    tenant_id = Column(Integer, ForeignKey("tenants.id"), nullable=False)
+    is_active = Column(Boolean, default=True, nullable=False)
+    created_at = Column(DateTime, default=datetime.utcnow)
+
+    # Relationships
+    tenant = relationship("Tenant", back_populates="users")