test(person): tighten generation null-clear coverage (#689)

Sara's QA concerns:

1. PersonControllerTest.updatePerson_returns200_whenGenerationNull was
   asymmetric — only checked status 200, no body assertion. Now also
   asserts `$.generation` is null in the JSON response, mirroring the
   in-range test's body check.

2. New full-stack PUT→DB→GET round-trip in PersonServiceIntegrationTest
   (updatePerson_clearGenerationToNull_readsBackNullFromDb) seeds a
   person with generation=3, calls updatePerson with generation=null,
   flushes the persistence context, and asserts the column reads back
   null from the DB. Without this we only had the mocked WebMvcTest
   boundary; nothing proved JPA actually wrote SQL NULL.

3. Sibling test (updatePerson_setGenerationToZero_readsBackZeroFromDb)
   pins the G 0 end-to-end so a primitive zero can't silently coerce
   to null anywhere along controller → service → JPA.

Refs #689

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

backend/src/test/java/org/raddatz/familienarchiv/person/PersonControllerTest.java

@@ -746,6 +746,10 @@ class PersonControllerTest {
     @Test
     @WithMockUser(authorities = "WRITE_ALL")
     void updatePerson_returns200_whenGenerationNull() throws Exception {
+        // Symmetric body assertion: the response must echo generation as null (not
+        // absent), so the frontend re-hydrates the "(none)" option after a clear.
+        // Without this, the in-range test below would be the only end-to-end proof
+        // that the field flows through the controller.
         Person saved = Person.builder().id(UUID.randomUUID()).firstName("Hans").lastName("Müller").build();
         when(personService.updatePerson(any(), any())).thenReturn(saved);
 
@@ -753,7 +757,8 @@ class PersonControllerTest {
                         .contentType(MediaType.APPLICATION_JSON)
                         .content("{\"firstName\":\"Hans\",\"lastName\":\"Müller\","
                                 + "\"personType\":\"PERSON\",\"generation\":null}"))
-                .andExpect(status().isOk());
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.generation").value(org.hamcrest.Matchers.nullValue()));
     }
 
     @Test

backend/src/test/java/org/raddatz/familienarchiv/person/PersonServiceIntegrationTest.java

@@ -124,6 +124,59 @@ class PersonServiceIntegrationTest {
         assertThat(personRepository.findById(target.getId())).isEmpty();
     }
 
+    // ─── generation full-stack round-trip (#689) ──────────────────────────────
+
+    @Test
+    void updatePerson_clearGenerationToNull_readsBackNullFromDb() {
+        // Sara's QA concern: pin the full PUT→DB→GET round-trip for the
+        // null-clear path. Without this we only have the WebMvcTest mocked
+        // boundary; nothing proved the JPA flush actually wrote SQL NULL.
+        Person seeded = personRepository.save(Person.builder()
+                .firstName("Hans").lastName("Raddatz")
+                .personType(PersonType.PERSON).generation(3).build());
+        entityManager.flush();
+        entityManager.clear();
+
+        PersonUpdateDTO dto = new PersonUpdateDTO();
+        dto.setPersonType(PersonType.PERSON);
+        dto.setFirstName("Hans");
+        dto.setLastName("Raddatz");
+        dto.setGeneration(null);
+
+        personService.updatePerson(seeded.getId(), dto);
+        entityManager.flush();
+        entityManager.clear();
+
+        Person reloaded = personRepository.findById(seeded.getId()).orElseThrow();
+        assertThat(reloaded.getGeneration()).isNull();
+    }
+
+    @Test
+    void updatePerson_setGenerationToZero_readsBackZeroFromDb() {
+        // Pin the G 0 case end-to-end. The form-action spec covers that 0
+        // doesn't get spread-dropped at the SvelteKit boundary; this test
+        // covers that the controller + service + JPA chain preserves the
+        // primitive zero (not coerced to null somewhere along the way).
+        Person seeded = personRepository.save(Person.builder()
+                .firstName("Walter").lastName("Raddatz")
+                .personType(PersonType.PERSON).build());
+        entityManager.flush();
+        entityManager.clear();
+
+        PersonUpdateDTO dto = new PersonUpdateDTO();
+        dto.setPersonType(PersonType.PERSON);
+        dto.setFirstName("Walter");
+        dto.setLastName("Raddatz");
+        dto.setGeneration(0);
+
+        personService.updatePerson(seeded.getId(), dto);
+        entityManager.flush();
+        entityManager.clear();
+
+        Person reloaded = personRepository.findById(seeded.getId()).orElseThrow();
+        assertThat(reloaded.getGeneration()).isEqualTo(0);
+    }
+
     @Test
     void deletePerson_detachesSentAndReceivedReferences_beforeDelete_noOrphan() {
         // A person referenced as BOTH a document sender and a document receiver must delete