Add templates/status/403.tmpl mirroring the 404 page design with
a centered error message and sign-in button for anonymous users.
New Forbidden() method on Context renders the styled 403 template
for HTML requests, falls back to plain text for API/non-HTML.
RepoAssignment now calls ctx.Forbidden() instead of raw HTTPError
for both anonymous and signed-in users without access.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>