refactor(TranscriptionBlock): document EAGER fetch rationale

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

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

@@ -35,6 +35,8 @@ public class TranscriptionBlock {
     @Column(columnDefinition = "TEXT")
     private String text;
 
+    // EAGER: mention set is bounded by block text length (typically < 20 entries).
+    // Switching back to LAZY requires callers to be inside an open Hibernate session.
     @ElementCollection(fetch = FetchType.EAGER)
     @CollectionTable(
             name = "transcription_block_mentioned_persons",