migration(transcription): add unique constraint on (block_id, person_id) sidecar

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

backend/src/main/resources/db/migration/V57__add_tbmp_unique_constraint.sql

@@ -0,0 +1,5 @@
+-- Prevent duplicate sidecar rows for the same (block, person) pair.
+-- @ElementCollection uses DELETE+INSERT per update so normal JPA writes can't
+-- create duplicates, but a raw-SQL import or concurrent bypass of JPA could.
+ALTER TABLE transcription_block_mentioned_persons
+    ADD CONSTRAINT uq_tbmp_block_person UNIQUE (block_id, person_id);

backend/src/test/java/org/raddatz/familienarchiv/service/PersonMentionPropagationListenerTest.java

@@ -26,6 +26,7 @@ import java.util.List;
 import java.util.UUID;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
 
 @DataJpaTest
 @AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@@ -195,6 +196,17 @@ class PersonMentionPropagationListenerTest {
         assertThat(first.getText()).contains("@Augusta Raddatz");
     }
 
+    @Test
+    void doesNotThrow_whenBlockTextIsNull() {
+        UUID personId = savedPersonId("Auguste", "Raddatz");
+        saveBlock(null, List.of(new PersonMention(personId, "Auguste Raddatz")));
+        em.clear();
+
+        assertThatCode(() -> listener.onPersonDisplayNameChanged(
+                new PersonDisplayNameChangedEvent(personId, "Auguste Raddatz", "Augusta Raddatz")))
+                .doesNotThrowAnyException();
+    }
+
     @Test
     void leavesUnrelatedBlockUntouched_whenNoSidecarReferencesPerson() {
         UUID personId = savedPersonId("Auguste", "Raddatz");