Spring Security's default Http403ForbiddenEntryPoint was returning 403
for all auth failures. Frontend clients treat 403 as a permission error
(not an auth error), so silent loops occurred instead of proper re-login.
Adding a custom AuthenticationEntryPoint that returns 401 makes clients
handle auth failures correctly (show login page on 401).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>