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/database/factories/UserFactory.php

@@ -18,7 +18,8 @@ final class UserFactory extends Factory
     public function definition(): array
     {
         return [
-            'name' => fake()->name(),
+            'first_name' => fake('nl_NL')->firstName(),
+            'last_name' => fake('nl_NL')->lastName(),
             'email' => fake()->unique()->safeEmail(),
             'email_verified_at' => now(),
             'password' => static::$password ??= Hash::make('password'),