Add comprehensive test suite and utilities for user functionality

This commit introduces a suite of tests for user models, schemas, CRUD operations, and authentication services. It also adds utilities for in-memory database setup to support these tests and updates environment settings for consistency.

backend/app/models/user.py

@@ -0,0 +1,19 @@
+from sqlalchemy import Column, String, JSON, Boolean
+
+from .base import Base, TimestampMixin, UUIDMixin
+
+
+class User(Base, UUIDMixin, TimestampMixin):
+    __tablename__ = 'users'
+
+    email = Column(String, unique=True, nullable=False, index=True)
+    password_hash = Column(String, nullable=False)
+    first_name = Column(String, nullable=False, default="user")
+    last_name = Column(String, nullable=True)
+    phone_number = Column(String)
+    is_active = Column(Boolean, default=True, nullable=False)
+    is_superuser = Column(Boolean, default=False, nullable=False)
+    preferences = Column(JSON)
+
+    def __repr__(self):
+        return f"<User {self.email}>"