feat(person): add nullable generation column to persons (#689)

Flyway V70: SMALLINT generation column with CHECK(0..10) and partial
index over non-null rows. Person.generation field surfaces it through
the JPA model. Pre-import rows and persons outside the curated family
graph legitimately stay null; the canonical importer (next commits)
back-fills via preferHuman so a human-edited value is never lost.

Refs #689

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

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

@@ -672,4 +672,39 @@ class PersonRepositoryTest {
 
         assertThat(slice.get(0).getDocumentCount()).isEqualTo(1);
     }
+
+    // ─── generation column (#689) ──────────────────────────────────────────────
+
+    @Test
+    void save_persistsGeneration_andFindByIdReturnsSameGeneration() {
+        Person person = Person.builder()
+                .firstName("Walter")
+                .lastName("Raddatz")
+                .generation(3)
+                .build();
+
+        Person saved = personRepository.save(person);
+        entityManager.flush();
+        entityManager.clear();
+
+        Optional<Person> found = personRepository.findById(saved.getId());
+        assertThat(found).isPresent();
+        assertThat(found.get().getGeneration()).isEqualTo(3);
+    }
+
+    @Test
+    void save_allowsNullGeneration_existingRowsRemainNull() {
+        Person person = Person.builder()
+                .firstName("Anonym")
+                .lastName("Person")
+                .build();
+
+        Person saved = personRepository.save(person);
+        entityManager.flush();
+        entityManager.clear();
+
+        Optional<Person> found = personRepository.findById(saved.getId());
+        assertThat(found).isPresent();
+        assertThat(found.get().getGeneration()).isNull();
+    }
 }