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/Api/V1/AssignablePersonsTest.php

@@ -199,8 +199,8 @@ class AssignablePersonsTest extends TestCase
         $shift1 = $this->createOpenShift();
         $shift2 = $this->createOpenShift(['festival_section_id' => $this->otherSection->id]);
 
-        $available = $this->createPerson(['name' => 'Anna Bakker']);
-        $conflicted = $this->createPerson(['name' => 'Bob Jansen']);
+        $available = $this->createPerson(['first_name' => 'Anna', 'last_name' => 'Bakker']);
+        $conflicted = $this->createPerson(['first_name' => 'Bob', 'last_name' => 'Jansen']);
 
         ShiftAssignment::factory()->create([
             'shift_id' => $shift1->id,