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/Person/PersonTest.php

@@ -119,17 +119,19 @@ class PersonTest extends TestCase
 
         $response = $this->postJson("/api/v1/events/{$this->event->id}/persons", [
             'crowd_type_id' => $this->crowdType->id,
-            'name' => 'Jan de Vries',
+            'first_name' => 'Jan',
+            'last_name' => 'de Vries',
             'email' => 'jan@test.nl',
             'phone' => '0612345678',
         ]);
 
         $response->assertCreated()
-            ->assertJson(['data' => ['name' => 'Jan de Vries', 'email' => 'jan@test.nl', 'status' => 'pending']]);
+            ->assertJson(['data' => ['first_name' => 'Jan', 'last_name' => 'de Vries', 'email' => 'jan@test.nl', 'status' => 'pending']]);
 
         $this->assertDatabaseHas('persons', [
             'event_id' => $this->event->id,
-            'name' => 'Jan de Vries',
+            'first_name' => 'Jan',
+            'last_name' => 'de Vries',
             'email' => 'jan@test.nl',
         ]);
     }