feat(conversations): filter person typeahead to correspondents of selected person

Closes #29

Backend:
- Add PersonRepository.findCorrespondents / findCorrespondentsWithFilter
  (native SQL, orders by shared document count DESC, limit 10)
- Add PersonService.findCorrespondents(personId, q) delegating to the
  correct repository method based on whether a query string is present
- Expose GET /api/persons/{id}/correspondents?q= in PersonController

Frontend:
- Add optional restrictToCorrespondentsOf prop to PersonTypeahead
- On focus with the prop set, fetch correspondents immediately (no typing
  required) — opens the dropdown showing top correspondents
- On input with the prop set, hit the correspondents endpoint with q= param
- Without the prop, keep existing /api/persons?q= behaviour unchanged
- Wire the prop bidirectionally in /conversations: sender restricts receiver
  and vice versa

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

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

@@ -10,6 +10,7 @@ import org.raddatz.familienarchiv.model.Person;
 import org.raddatz.familienarchiv.repository.PersonRepository;
 import org.springframework.web.server.ResponseStatusException;
 
+import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
 
@@ -158,6 +159,41 @@ class PersonServiceTest {
         assertThat(result.getDeathYear()).isEqualTo(1900);
     }
 
+    // ─── findCorrespondents ──────────────────────────────────────────────────
+
+    @Test
+    void findCorrespondents_delegatesToRepository_withoutFilter() {
+        UUID personId = UUID.randomUUID();
+        List<Person> expected = List.of(Person.builder().id(UUID.randomUUID()).firstName("Anna").lastName("Muster").build());
+        when(personRepository.findCorrespondents(personId)).thenReturn(expected);
+
+        assertThat(personService.findCorrespondents(personId, null)).isEqualTo(expected);
+        verify(personRepository).findCorrespondents(personId);
+        verify(personRepository, never()).findCorrespondentsWithFilter(any(), any());
+    }
+
+    @Test
+    void findCorrespondents_delegatesToRepository_withFilter() {
+        UUID personId = UUID.randomUUID();
+        List<Person> expected = List.of(Person.builder().id(UUID.randomUUID()).firstName("Anna").lastName("Muster").build());
+        when(personRepository.findCorrespondentsWithFilter(personId, "Anna")).thenReturn(expected);
+
+        assertThat(personService.findCorrespondents(personId, "Anna")).isEqualTo(expected);
+        verify(personRepository).findCorrespondentsWithFilter(personId, "Anna");
+        verify(personRepository, never()).findCorrespondents(any());
+    }
+
+    @Test
+    void findCorrespondents_delegatesToRepository_withBlankFilter() {
+        UUID personId = UUID.randomUUID();
+        when(personRepository.findCorrespondents(personId)).thenReturn(List.of());
+
+        personService.findCorrespondents(personId, "   ");
+
+        verify(personRepository).findCorrespondents(personId);
+        verify(personRepository, never()).findCorrespondentsWithFilter(any(), any());
+    }
+
     // ─── mergePersons ─────────────────────────────────────────────────────────
 
     @Test