PersonMentionPropagationListener rewrites @DisplayName tokens on person rename.
Under the new design, displayName is archival (what the transcriber typed), so
the listener would corrupt transcriptions rather than correct them.
Deletes PersonMentionPropagationListener, PersonDisplayNameChangedEvent, and the
optimistic-lock catch path in PersonService.updatePerson. Removes PERSON_RENAME_CONFLICT
from ErrorCode and all tests that exercised the now-deleted code path.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>