fix(transcription): eliminate N+1 lazy load in propagation listener

Switch from findByMentionedPersons_PersonId (derived query, returns blocks with
LAZY mentionedPersons) to findByPersonIdWithMentionsFetched (JOIN FETCH, loads
full collections in one round-trip). 200-block propagation: from 201 queries to 2.
Add @Transactional comment documenting join-transaction semantics.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/service/PersonMentionPropagationListener.java

@@ -35,10 +35,10 @@ public class PersonMentionPropagationListener {
     private final TranscriptionBlockRepository blockRepository;
 
     @EventListener
-    @Transactional
+    @Transactional // Joins publisher's transaction — async switch requires @TransactionalEventListener(AFTER_COMMIT)
     public void onPersonDisplayNameChanged(PersonDisplayNameChangedEvent event) {
         List<TranscriptionBlock> blocks =
-                blockRepository.findByMentionedPersons_PersonId(event.personId());
+                blockRepository.findByPersonIdWithMentionsFetched(event.personId());
         if (blocks.isEmpty()) {
             return;
         }