fix(search): handle null firstName in all search queries

Use COALESCE to convert null firstName to empty string in:
- PersonRepository.searchByName (JPQL)
- PersonRepository.searchWithDocumentCount (native SQL)
- PersonRepository.findCorrespondentsWithFilter (native SQL)
- DocumentSpecifications.hasText (Criteria API, sender + receiver)

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

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

@@ -440,4 +440,26 @@ class PersonRepositoryTest {
         assertThat(results).hasSize(1);
         assertThat(results.get(0).getLastName()).isEqualTo("Cram");
     }
+
+    // ─── null firstName handling ────────────────────────────────────────────
+
+    @Test
+    void searchByName_findsPersonWithNullFirstName() {
+        personRepository.save(Person.builder().lastName("Gesellschafter des Verlages").build());
+
+        List<Person> result = personRepository.searchByName("Gesellschafter");
+
+        assertThat(result).hasSize(1);
+        assertThat(result.get(0).getLastName()).isEqualTo("Gesellschafter des Verlages");
+    }
+
+    @Test
+    void searchWithDocumentCount_findsPersonWithNullFirstName() {
+        personRepository.save(Person.builder().lastName("Gesellschafter des Verlages").build());
+
+        List<PersonSummaryDTO> result = personRepository.searchWithDocumentCount("Gesellschafter");
+
+        assertThat(result).hasSize(1);
+        assertThat(result.get(0).getLastName()).isEqualTo("Gesellschafter des Verlages");
+    }
 }