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/pages/dashboard/index.vue

@@ -16,7 +16,7 @@ const stats = [
     <VCard class="mb-6">
       <VCardText>
         <h4 class="text-h4 mb-1">
-          Welkom, {{ authStore.user?.name ?? 'gebruiker' }}! 👋
+          Welkom, {{ authStore.user?.full_name ?? 'gebruiker' }}! 👋
         </h4>
         <p class="text-body-1 mb-0">
           {{ authStore.currentOrganisation?.name ?? 'Geen organisatie geselecteerd' }}