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>

apps/app/src/components/members/EditMemberRoleDialog.vue

@@ -80,7 +80,7 @@ function onSubmit() {
           <VRow class="mb-4">
             <VCol cols="12">
               <div class="text-body-1">
-                <strong>{{ member.name }}</strong>
+                <strong>{{ member.full_name }}</strong>
               </div>
               <div class="text-body-2 text-disabled">
                 {{ member.email }}