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/Http/Requests/Api/V1/StoreCompanyRequest.php

@@ -19,7 +19,8 @@ final class StoreCompanyRequest extends FormRequest
         return [
             'name' => ['required', 'string', 'max:100'],
             'type' => ['required', 'in:supplier,partner,agency,venue,other'],
-            'contact_name' => ['nullable', 'string', 'max:100'],
+            'contact_first_name' => ['nullable', 'string', 'max:100'],
+            'contact_last_name' => ['nullable', 'string', 'max:100'],
             'contact_email' => ['nullable', 'email', 'max:100'],
             'contact_phone' => ['nullable', 'string', 'max:30'],
         ];