feat(transcription): TranscriptionBlock.mentionedPersons sidecar field

@ElementCollection(LAZY) on List<PersonMention>, mapped to V56's
transcription_block_mentioned_persons via explicit @CollectionTable that
matches the migration name byte-for-byte (immune to Hibernate naming-strategy
changes). @Builder.Default keeps the field initialized to an empty list, so
existing transcription block construction stays untouched.

Refs #362

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -7,6 +7,8 @@ import org.hibernate.annotations.CreationTimestamp;
 import org.hibernate.annotations.UpdateTimestamp;
 
 import java.time.LocalDateTime;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.UUID;
 
 @Entity
@@ -33,6 +35,14 @@ public class TranscriptionBlock {
     @Column(columnDefinition = "TEXT")
     private String text;
 
+    @ElementCollection(fetch = FetchType.LAZY)
+    @CollectionTable(
+            name = "transcription_block_mentioned_persons",
+            joinColumns = @JoinColumn(name = "block_id"))
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    @Builder.Default
+    private List<PersonMention> mentionedPersons = new ArrayList<>();
+
     @Column(length = 200)
     private String label;