feat: add dev-login endpoint

[phytter]

Summary

• Add /dev-login POST endpoint that allows authentication via email/name without Google OAuth, restricted to debug mode only (APP_DEBUG=True)
• Add DevLoginData DTO and dev_login_service business logic that creates or updates users
• Add tests covering new user creation, existing user login, and forbidden access when debug mode is off

Test plan

• test_dev_login_success_new_user — creates a new user and returns access token
• test_dev_login_success_existing_user — logs in existing user and updates last_login
• test_dev_login_forbidden_when_not_debug — returns 403 when APP_DEBUG is false

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