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/tests/Feature/Company/CompanyTest.php

@@ -100,7 +100,8 @@ class CompanyTest extends TestCase
         $response = $this->postJson("/api/v1/organisations/{$this->organisation->id}/companies", [
             'name' => 'Catering Janssen',
             'type' => 'supplier',
-            'contact_name' => 'Jan Janssen',
+            'contact_first_name' => 'Jan',
+            'contact_last_name' => 'Janssen',
             'contact_email' => 'jan@janssen.nl',
             'contact_phone' => '+31612345678',
         ]);