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/Event/FestivalEventTest.php

@@ -201,14 +201,16 @@ class FestivalEventTest extends TestCase
         Person::factory()->create([
             'event_id' => $this->festival->id,
             'crowd_type_id' => $this->crowdType->id,
-            'name' => 'Jan Festivalmedewerker',
+            'first_name' => 'Jan',
+            'last_name' => 'Festivalmedewerker',
         ]);
 
         // Create a person on the sub-event
         Person::factory()->create([
             'event_id' => $this->subEvent->id,
             'crowd_type_id' => $this->crowdType->id,
-            'name' => 'Piet Dagvrijwilliger',
+            'first_name' => 'Piet',
+            'last_name' => 'Dagvrijwilliger',
         ]);
 
         Sanctum::actingAs($this->orgAdmin);
@@ -218,7 +220,7 @@ class FestivalEventTest extends TestCase
 
         $response->assertOk();
 
-        $personNames = collect($response->json('data'))->pluck('name')->all();
+        $personNames = collect($response->json('data'))->pluck('full_name')->all();
         $this->assertContains('Jan Festivalmedewerker', $personNames);
         $this->assertNotContains('Piet Dagvrijwilliger', $personNames);
     }