Refactor database module and add testing utilities

Simplify database module by re-organizing engine creation, session handling, and removing redundant methods. Introduce SQLite compatibility for testing and add a utility module for test database setup and teardown. Integrate initial unit tests for user models and update dependencies for security and testing.

backend/tests/models/test_user.py

@@ -0,0 +1,29 @@
+# tests/models/test_user.py
+import uuid
+from app.models.user import User
+
+def test_create_user(db_session):
+    # Arrange
+    user_id = uuid.uuid4()
+    new_user = User(
+        id=user_id,
+        email="test@example.com",
+        password_hash="hashedpassword",
+        first_name="Test",
+        last_name="User",
+        phone_number="1234567890",
+        is_active=True,
+        is_superuser=False,
+        preferences={"theme": "dark"},
+    )
+    db_session.add(new_user)
+
+    # Act
+    db_session.commit()
+    created_user = db_session.query(User).filter_by(email="test@example.com").first()
+
+    # Assert
+    assert created_user is not None
+    assert created_user.email == "test@example.com"
+    assert created_user.first_name == "Test"
+    assert created_user.preferences == {"theme": "dark"}