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/Auth/LoginTest.php

@@ -24,7 +24,7 @@ class LoginTest extends TestCase
         $response->assertOk()
             ->assertJsonStructure([
                 'success',
-                'data' => ['user' => ['id', 'name', 'email'], 'token'],
+                'data' => ['user' => ['id', 'first_name', 'last_name', 'full_name', 'email'], 'token'],
                 'message',
             ])
             ->assertJson(['success' => true]);

api/tests/Feature/Auth/MeTest.php

@@ -35,7 +35,7 @@ class MeTest extends TestCase
             ->assertJsonStructure([
                 'success',
                 'data' => [
-                    'id', 'name', 'email', 'timezone', 'locale',
+                    'id', 'first_name', 'last_name', 'full_name', 'email', 'timezone', 'locale',
                     'organisations', 'app_roles', 'permissions',
                 ],
             ]);