feat(person): route generation through service write paths (#689)

- fromCanonical writes the imported generation into a new Person row.
- mergeCanonical routes existing/canonical generation through the
  existing preferHuman(Integer, Integer) overload so a human-edited
  value is never overwritten on re-import (ADR-025).
- updatePerson writes generation verbatim from the form DTO so a human
  can clear it back to null — same shape as birthYear/deathYear.
- createPerson(PersonUpdateDTO) writes generation so /persons/new flow
  doesn't silently drop a selected G value on create.

Pinned with five tests covering the four write paths plus the
documenting test that captures preferHuman's known limitation
(explicit human null is overwritten by a non-null canonical value —
same as birthYear/deathYear, deferred to a future helper rework if it
ever produces a user-visible bug).

Refs #689

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

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

@@ -177,6 +177,7 @@ public class PersonService {
                 .notes(blankToNull(cmd.notes()))
                 .birthYear(cmd.birthYear())
                 .deathYear(cmd.deathYear())
+                .generation(cmd.generation())
                 .familyMember(cmd.familyMember())
                 .personType(cmd.personType() == null ? PersonType.PERSON : cmd.personType())
                 .provisional(cmd.provisional())
@@ -200,6 +201,7 @@ public class PersonService {
         existing.setNotes(preferHuman(existing.getNotes(), cmd.notes()));
         existing.setBirthYear(preferHuman(existing.getBirthYear(), cmd.birthYear()));
         existing.setDeathYear(preferHuman(existing.getDeathYear(), cmd.deathYear()));
+        existing.setGeneration(preferHuman(existing.getGeneration(), cmd.generation()));
         if (cmd.personType() != null && existing.getPersonType() == PersonType.PERSON) {
             existing.setPersonType(cmd.personType());
         }
@@ -254,6 +256,7 @@ public class PersonService {
                 .notes(dto.getNotes() == null || dto.getNotes().isBlank() ? null : dto.getNotes().trim())
                 .birthYear(dto.getBirthYear())
                 .deathYear(dto.getDeathYear())
+                .generation(dto.getGeneration())
                 .build();
         return personRepository.save(person);
     }
@@ -286,6 +289,9 @@ public class PersonService {
         person.setNotes(dto.getNotes() == null || dto.getNotes().isBlank() ? null : dto.getNotes().trim());
         person.setBirthYear(dto.getBirthYear());
         person.setDeathYear(dto.getDeathYear());
+        // Form path: a human can clear generation back to null. Unlike the importer
+        // which routes through preferHuman, we write the DTO value verbatim.
+        person.setGeneration(dto.getGeneration());
         return personRepository.save(person);
     }