fix(routing): resolve user codes to handles on 404
When visiting /ac25namuc (a user code), the system now checks the
permahandle API before showing 404. If the code resolves to a handle,
it redirects to @handle instead. Previously these URLs always 404'd
because user codes aren't piece names.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>