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/Models/User.php

@@ -24,7 +24,8 @@ final class User extends Authenticatable
     use SoftDeletes;
 
     protected $fillable = [
-        'name',
+        'first_name',
+        'last_name',
         'email',
         'password',
         'timezone',
@@ -32,6 +33,16 @@ final class User extends Authenticatable
         'avatar',
     ];
 
+    public function getFullNameAttribute(): string
+    {
+        return trim("{$this->first_name} {$this->last_name}");
+    }
+
+    public function getNameAttribute(): string
+    {
+        return $this->full_name;
+    }
+
     protected $hidden = [
         'password',
         'remember_token',