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/pytest.ini

@@ -0,0 +1,10 @@
+[pytest]
+env =
+    IS_TEST=True
+testpaths = tests
+python_files = test_*.py
+addopts = --disable-warnings
+markers =
+    sqlite: marks tests that should run on SQLite (mocked).
+    postgres: marks tests that require a real PostgreSQL database.
+asyncio_default_fixture_loop_scope = function