Backend: migration 007 adds a case-insensitive unique index on user names
per event. join endpoint returns 409 conflict when the name is taken.
find_by_event_and_name uses LOWER() for case-insensitive recovery.
Frontend: join page handles 409 with a name-taken view — amber warning,
name-choice tips, inline PIN recovery form, and "Anderen Namen wählen"
button. Test guide updated with Steps 8 and 9.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
