Add comprehensive unit tests for auth endpoints

22 tests covering:
- Helper functions (_domain_of, _check_domain_allowed, _default_tier, _is_admin_email)
- POST /register (success, duplicate email, domain restriction)
- POST /login (success, user not found, wrong password, inactive, admin auto-promotion)
- POST /google (existing user, new user, unverified email, domain restriction)
- GET /google/authorize-url (correct URL construction)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 <!-- version list -->
 
+## [Unreleased]
+
+### Added
+
+- Unit tests for authentication endpoints (`test_auth.py`): 22 tests covering register, login, Google OAuth, authorize-url, and helper functions
+
 ## v0.6.0 (2026-03-29)
 
 ### Chores