test(transcription): round-trip TranscriptionBlock.mentionedPersons

@DataJpaTest + Testcontainers exercises the V56 migration plus the
@ElementCollection wiring end-to-end. Saves a block with two PersonMention
entries, clears the persistence context, reloads, asserts both entries
return with their personId + displayName intact. Second test guards the
@Builder.Default — a block without explicit mentions reloads with an empty
list, not null.

Refs #362

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

backend/src/test/java/org/raddatz/familienarchiv/repository/TranscriptionBlockMentionsRepositoryTest.java

@@ -0,0 +1,95 @@
+package org.raddatz.familienarchiv.repository;
+
+import jakarta.persistence.EntityManager;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.raddatz.familienarchiv.PostgresContainerConfig;
+import org.raddatz.familienarchiv.config.FlywayConfig;
+import org.raddatz.familienarchiv.model.Document;
+import org.raddatz.familienarchiv.model.DocumentAnnotation;
+import org.raddatz.familienarchiv.model.DocumentStatus;
+import org.raddatz.familienarchiv.model.PersonMention;
+import org.raddatz.familienarchiv.model.TranscriptionBlock;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.jdbc.test.autoconfigure.AutoConfigureTestDatabase;
+import org.springframework.boot.data.jpa.test.autoconfigure.DataJpaTest;
+import org.springframework.context.annotation.Import;
+
+import java.util.List;
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@DataJpaTest
+@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
+@Import({PostgresContainerConfig.class, FlywayConfig.class})
+class TranscriptionBlockMentionsRepositoryTest {
+
+    @Autowired TranscriptionBlockRepository blockRepository;
+    @Autowired DocumentRepository documentRepository;
+    @Autowired AnnotationRepository annotationRepository;
+    @Autowired EntityManager em;
+
+    private UUID documentId;
+    private UUID annotationId;
+
+    @BeforeEach
+    void setUp() {
+        Document doc = documentRepository.save(Document.builder()
+                .title("Letter")
+                .originalFilename("letter.pdf")
+                .status(DocumentStatus.UPLOADED)
+                .build());
+        documentId = doc.getId();
+
+        DocumentAnnotation annotation = annotationRepository.save(DocumentAnnotation.builder()
+                .documentId(documentId)
+                .pageNumber(1)
+                .x(0.1).y(0.2).width(0.3).height(0.4)
+                .color("#00C7B1")
+                .build());
+        annotationId = annotation.getId();
+    }
+
+    @Test
+    void mentionedPersons_roundTripsTwoEntries() {
+        UUID auguste = UUID.randomUUID();
+        UUID hermann = UUID.randomUUID();
+
+        TranscriptionBlock saved = blockRepository.saveAndFlush(TranscriptionBlock.builder()
+                .annotationId(annotationId)
+                .documentId(documentId)
+                .text("Liebe Tante @Auguste Raddatz, Onkel @Hermann Müller schreibt …")
+                .sortOrder(0)
+                .mentionedPersons(List.of(
+                        new PersonMention(auguste, "Auguste Raddatz"),
+                        new PersonMention(hermann, "Hermann Müller")
+                ))
+                .build());
+
+        em.clear();
+
+        TranscriptionBlock reloaded = blockRepository.findById(saved.getId()).orElseThrow();
+
+        assertThat(reloaded.getMentionedPersons())
+                .extracting(PersonMention::getPersonId, PersonMention::getDisplayName)
+                .containsExactlyInAnyOrder(
+                        org.assertj.core.groups.Tuple.tuple(auguste, "Auguste Raddatz"),
+                        org.assertj.core.groups.Tuple.tuple(hermann, "Hermann Müller"));
+    }
+
+    @Test
+    void mentionedPersons_defaultsToEmptyList_whenNotSet() {
+        TranscriptionBlock saved = blockRepository.saveAndFlush(TranscriptionBlock.builder()
+                .annotationId(annotationId)
+                .documentId(documentId)
+                .text("Plain text without mentions")
+                .sortOrder(0)
+                .build());
+
+        em.clear();
+
+        TranscriptionBlock reloaded = blockRepository.findById(saved.getId()).orElseThrow();
+        assertThat(reloaded.getMentionedPersons()).isEmpty();
+    }
+}