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/app/Http/Controllers/Api/V1/InvitationController.php

@@ -64,7 +64,9 @@ final class InvitationController extends Controller
         return $this->success([
             'user' => [
                 'id' => $user->id,
-                'name' => $user->name,
+                'first_name' => $user->first_name,
+                'last_name' => $user->last_name,
+                'full_name' => $user->full_name,
                 'email' => $user->email,
             ],
             'token' => $sanctumToken,

api/app/Http/Controllers/Api/V1/ShiftAssignmentController.php

@@ -145,7 +145,8 @@ final class ShiftAssignmentController extends Controller
         $persons = Person::where('event_id', $festivalEventId)
             ->where('status', PersonStatus::APPROVED)
             ->with('crowdType')
-            ->orderBy('name')
+            ->orderBy('first_name')
+            ->orderBy('last_name')
             ->get();
 
         // Batch: tags for all persons with user_id
@@ -184,7 +185,9 @@ final class ShiftAssignmentController extends Controller
 
                 return [
                     'id' => $person->id,
-                    'name' => $person->name,
+                    'first_name' => $person->first_name,
+                    'last_name' => $person->last_name,
+                    'full_name' => $person->full_name,
                     'email' => $person->email,
                     'status' => $person->status,
                     'crowd_type' => $person->crowdType ? [
@@ -222,7 +225,7 @@ final class ShiftAssignmentController extends Controller
             ->sortBy([
                 ['already_assigned', 'asc'],
                 ['is_available', 'desc'],
-                ['name', 'asc'],
+                ['first_name', 'asc'],
             ])
             ->values();