Add user model to fix import error on Render

Author: Uttam Singh

backend/app/models/user.py

@@ -0,0 +1,12 @@
+from sqlalchemy import Column, Integer, String
+from sqlalchemy.orm import declarative_base
+
+Base = declarative_base()
+
+class User(Base):
+    __tablename__ = "users"
+
+    id = Column(Integer, primary_key=True, index=True)
+    username = Column(String(100), unique=True, nullable=False)
+    password = Column(String(255), nullable=False)
+    role = Column(String(50), default="member")