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/Services/VolunteerRegistrationService.php

@@ -50,7 +50,8 @@ final class VolunteerRegistrationService
                 [
                     'user_id' => $user?->id,
                     'crowd_type_id' => $volunteerCrowdType->id,
-                    'name' => $user?->name ?? $validated['name'],
+                    'first_name' => $user?->first_name ?? $validated['first_name'],
+                    'last_name' => $user?->last_name ?? $validated['last_name'],
                     'phone' => $validated['phone'] ?? null,
                     'status' => PersonStatus::PENDING,
                     'custom_fields' => [
@@ -119,7 +120,7 @@ final class VolunteerRegistrationService
             return;
         }
 
-        if ($existing->status !== PersonStatus::REJECTED) {
+        if ($existing->status !== PersonStatus::REJECTED->value) {
             throw ValidationException::withMessages([
                 'email' => ['Already registered for this event.'],
             ]);