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/api/main.py

@@ -0,0 +1,6 @@
+from fastapi import APIRouter
+
+from app.api.routes import auth
+
+api_router = APIRouter()
+api_router.include_router(auth.router, prefix="/auth", tags=["auth"])