Carries personId + oldDisplayName + newDisplayName so transcription-side
listeners can rewrite block.text and sidecar entries when a person is
renamed. First custom application event in this codebase — the only prior
@EventListener consumes Spring's built-in ApplicationReadyEvent. Class doc
sets the convention for future cross-domain decoupling.
Refs #362
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>