feat: split name into first_name + last_name across users, persons, and companies

Cross-cutting migration affecting the entire stack:
- Database: 3 migrations splitting name columns with data migration
- Models: first_name/last_name on User, Person; contact_first_name/contact_last_name on Company; backward-compatible name accessors
- API: all resources return first_name, last_name, full_name; assignablePersons endpoint updated
- Requests: validation rules updated for all person/user/company forms
- Services: VolunteerRegistrationService, ShiftAssignmentService, InvitationService updated
- Frontend: TypeScript types, Zod schemas, all forms split into Voornaam/Achternaam fields
- Display: all person/user name references use full_name; initials use first_name[0]+last_name[0]
- Tests: all 371 tests passing
- Docs: SCHEMA.md and API.md updated

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Services/ShiftAssignmentService.php

@@ -102,7 +102,7 @@ final class ShiftAssignmentService
                     ->performedOn($existing)
                     ->withProperties([
                         'previous_status' => $previousStatus,
-                        'person_name' => $person->name,
+                        'person_name' => $person->full_name,
                     ])
                     ->log('shift_assignment.reactivated');
 
@@ -124,7 +124,7 @@ final class ShiftAssignmentService
                         'filled_slots' => $filledSlots,
                         'slots_total' => $shift->slots_total,
                         'person_id' => $person->id,
-                        'person_name' => $person->name,
+                        'person_name' => $person->full_name,
                     ])
                     ->log('shift.overbooked_assignment');
             }