feat(person): PersonDisplayNameChangedEvent record

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>

backend/src/main/java/org/raddatz/familienarchiv/model/PersonDisplayNameChangedEvent.java

@@ -0,0 +1,23 @@
+package org.raddatz.familienarchiv.model;
+
+import java.util.UUID;
+
+/**
+ * Published by PersonService when a save changes Person.getDisplayName() — i.e.
+ * any mutation to the fields that DisplayNameFormatter consumes (title,
+ * firstName, lastName). Listeners on the transcription side rewrite block text
+ * and sidecar entries that reference the old name.
+ *
+ * <p>This is the first custom application event in the codebase. The previous
+ * only listener (OcrTrainingService.recoverOrphanedRuns) listens to Spring's
+ * built-in ApplicationReadyEvent. Future cross-domain decoupling should follow
+ * the same shape: record-typed event in model/, listener in the consuming
+ * domain's service/ package, synchronous @EventListener inside the publisher's
+ * transaction unless the workload genuinely needs to defer.
+ */
+public record PersonDisplayNameChangedEvent(
+        UUID personId,
+        String oldDisplayName,
+        String newDisplayName
+) {
+}