test(transcription): partial-name collision does not corrupt unrenamed mention

Block contains both @Hans-Peter Müller and @Hans Müller; the listener fires
a rename for Hans Müller → Hans Schmidt. The simple replace("@" + old,
"@" + new) hinges on the leading @-and-space anchor: "@Hans Müller" does
not appear inside "@Hans-Peter Müller" (hyphen interrupts), so only the
standalone mention rewrites. Sidecar mirrors the same — Hans Müller's
entry flips to Hans Schmidt while Hans-Peter Müller's entry is preserved.

Refs #362

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

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

@@ -96,6 +96,32 @@ class PersonMentionPropagationListenerTest {
                 .containsExactly("Augusta Raddatz");
     }
 
+    @Test
+    void doesNotMatchPartialName_whenAnotherMentionShares_a_substring_with_renamed_person() {
+        UUID hansPeterId = savedPersonId("Hans-Peter", "Müller");
+        UUID hansId = savedPersonId("Hans", "Müller");
+        TranscriptionBlock saved = saveBlock(
+                "Heute hat @Hans-Peter Müller wieder mit @Hans Müller gesprochen.",
+                List.of(
+                        new PersonMention(hansPeterId, "Hans-Peter Müller"),
+                        new PersonMention(hansId, "Hans Müller")));
+        em.clear();
+
+        listener.onPersonDisplayNameChanged(
+                new PersonDisplayNameChangedEvent(hansId, "Hans Müller", "Hans Schmidt"));
+        blockRepository.flush();
+        em.clear();
+
+        TranscriptionBlock reloaded = blockRepository.findById(saved.getId()).orElseThrow();
+        assertThat(reloaded.getText())
+                .isEqualTo("Heute hat @Hans-Peter Müller wieder mit @Hans Schmidt gesprochen.");
+        assertThat(reloaded.getMentionedPersons())
+                .extracting(PersonMention::getPersonId, PersonMention::getDisplayName)
+                .containsExactlyInAnyOrder(
+                        org.assertj.core.groups.Tuple.tuple(hansPeterId, "Hans-Peter Müller"),
+                        org.assertj.core.groups.Tuple.tuple(hansId, "Hans Schmidt"));
+    }
+
     @Test
     void leavesUnrelatedBlockUntouched_whenNoSidecarReferencesPerson() {
         UUID personId = savedPersonId("Auguste", "Raddatz");