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/main/java/org/raddatz/familienarchiv/controller/PersonController.java

@@ -34,6 +34,13 @@ public class PersonController {
         return personService.getById(id);
     }
 
+    @GetMapping("/{id}/correspondents")
+    public ResponseEntity<List<Person>> getCorrespondents(
+            @PathVariable UUID id,
+            @RequestParam(required = false) String q) {
+        return ResponseEntity.ok(personService.findCorrespondents(id, q));
+    }
+
     @GetMapping("/{id}/documents")
     public List<Document> getPersonDocuments(@PathVariable UUID id) {
         return documentService.getDocumentsBySender(id);

backend/src/main/java/org/raddatz/familienarchiv/repository/PersonRepository.java

@@ -28,6 +28,51 @@ public interface PersonRepository extends JpaRepository<Person, UUID> {
     // Lookup by full alias string, used during ODS mass import
     Optional<Person> findByAliasIgnoreCase(String alias);
 
+    // --- Correspondent queries ---
+
+    @Query(value = """
+            SELECT p.* FROM persons p
+            INNER JOIN (
+                SELECT dr.person_id AS other_id, d.id AS doc_id
+                FROM documents d
+                JOIN document_receivers dr ON dr.document_id = d.id
+                WHERE d.sender_id = :personId
+                UNION ALL
+                SELECT d.sender_id AS other_id, d.id AS doc_id
+                FROM documents d
+                JOIN document_receivers dr ON dr.document_id = d.id
+                WHERE dr.person_id = :personId AND d.sender_id IS NOT NULL
+            ) shared ON shared.other_id = p.id
+            WHERE p.id != :personId
+            GROUP BY p.id
+            ORDER BY COUNT(DISTINCT shared.doc_id) DESC
+            LIMIT 10
+            """, nativeQuery = true)
+    List<Person> findCorrespondents(@Param("personId") UUID personId);
+
+    @Query(value = """
+            SELECT p.* FROM persons p
+            INNER JOIN (
+                SELECT dr.person_id AS other_id, d.id AS doc_id
+                FROM documents d
+                JOIN document_receivers dr ON dr.document_id = d.id
+                WHERE d.sender_id = :personId
+                UNION ALL
+                SELECT d.sender_id AS other_id, d.id AS doc_id
+                FROM documents d
+                JOIN document_receivers dr ON dr.document_id = d.id
+                WHERE dr.person_id = :personId AND d.sender_id IS NOT NULL
+            ) shared ON shared.other_id = p.id
+            WHERE p.id != :personId
+            AND (LOWER(CONCAT(p.first_name,' ',p.last_name)) LIKE LOWER(CONCAT('%',:q,'%'))
+                 OR LOWER(CONCAT(p.last_name,' ',p.first_name)) LIKE LOWER(CONCAT('%',:q,'%'))
+                 OR LOWER(p.alias) LIKE LOWER(CONCAT('%',:q,'%')))
+            GROUP BY p.id
+            ORDER BY COUNT(DISTINCT shared.doc_id) DESC
+            LIMIT 10
+            """, nativeQuery = true)
+    List<Person> findCorrespondentsWithFilter(@Param("personId") UUID personId, @Param("q") String q);
+
     // --- Merge helpers (native SQL to bypass JPA entity layer) ---
 
     @Modifying

backend/src/main/java/org/raddatz/familienarchiv/service/PersonService.java

@@ -31,6 +31,13 @@ public class PersonService {
                 .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "Person nicht gefunden"));
     }
 
+    public List<Person> findCorrespondents(UUID personId, String q) {
+        if (q != null && !q.isBlank()) {
+            return personRepository.findCorrespondentsWithFilter(personId, q);
+        }
+        return personRepository.findCorrespondents(personId);
+    }
+
     public List<Person> getAllById(List<UUID> ids) {
         return personRepository.findAllById(ids);
     }

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