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/base.py

@@ -0,0 +1,20 @@
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import Column, DateTime
+from sqlalchemy.dialects.postgresql import UUID
+
+# noinspection PyUnresolvedReferences
+from app.core.database import Base
+
+
+class TimestampMixin:
+    """Mixin to add created_at and updated_at timestamps to models"""
+    created_at = Column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc), nullable=False)
+    updated_at = Column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc),
+                        onupdate=lambda: datetime.now(timezone.utc), nullable=False)
+
+
+class UUIDMixin:
+    """Mixin to add UUID primary keys to models"""
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)