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/__init__.py

@@ -0,0 +1,14 @@
+"""
+Models package initialization.
+Imports all models to ensure they're registered with SQLAlchemy.
+"""
+# First import Base to avoid circular imports
+from app.core.database import Base
+from .base import TimestampMixin, UUIDMixin
+
+# Import user model
+from .user import User
+__all__ = [
+    'Base', 'TimestampMixin', 'UUIDMixin',
+    'User',
+]