refactor(person): unify fill-blank under preferHuman and clarify rowId trap

Unify birthYear/deathYear fill-blank logic under an Integer preferHuman overload so
every canonical field uses one self-documenting precedence idiom, and add a guard
test pinning year fill-blank vs human-edit preservation. Add a comment in
PersonTreeImporter.createRelationships noting the relationship node's personId field
carries a tree rowId, not a person slug.

Refs #669

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

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

@@ -97,6 +97,26 @@ class PersonImportUpsertTest {
         assertThat(result.getNotes()).isEqualTo("Nichte von Herbert");
     }
 
+    @Test
+    void upsertBySourceRef_fillsBlankYears_butPreservesHumanEditedYears_onReimport() {
+        // Existing has a human-set birthYear and a blank deathYear.
+        Person existing = Person.builder()
+                .id(UUID.randomUUID()).sourceRef("clara-cram")
+                .lastName("Cram").birthYear(1890).deathYear(null).build();
+        when(personRepository.findBySourceRef("clara-cram")).thenReturn(Optional.of(existing));
+        when(personRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        PersonUpsertCommand cmd = PersonUpsertCommand.builder()
+                .sourceRef("clara-cram").lastName("Cram")
+                .birthYear(1888).deathYear(1965)
+                .personType(PersonType.PERSON).provisional(false).build();
+
+        Person result = personService.upsertBySourceRef(cmd);
+
+        assertThat(result.getBirthYear()).isEqualTo(1890); // human value kept
+        assertThat(result.getDeathYear()).isEqualTo(1965); // blank filled from canonical
+    }
+
     @Test
     void upsertBySourceRef_neverFlipsProvisionalBackToTrue_onceHumanConfirmed() {
         // A human confirmed this provisional importer-created person (provisional -> false).