[BE-58] Add unit tests for AuthService

[mftee]

Overview

AuthService handles all credential validation and token generation but has zero test coverage. A regression in hashing or token logic would not be caught.

Test Cases to Cover

• register() — creates user with bcrypt-hashed password, rejects duplicate email with 409
• login() — returns tokens on valid credentials, throws 401 on wrong password, throws 401 on unknown email
• refreshToken() — returns new access token for valid refresh token, throws 401 for tampered token
• handleOAuthLogin() — creates new user on first OAuth login, returns existing user on subsequent logins

File

src/auth/auth.service.spec.ts