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/persons/PersonDetailPanel.vue

@@ -103,11 +103,11 @@ function onBlacklistToggle(val: boolean | null) {
               color="primary"
               variant="tonal"
             >
-              <span class="text-h6">{{ getInitials(person.name) }}</span>
+              <span class="text-h6">{{ getInitials(person.full_name) }}</span>
             </VAvatar>
             <div>
               <h5 class="text-h5 mb-1">
-                {{ person.name }}
+                {{ person.full_name }}
               </h5>
               <p class="text-body-2 text-disabled mb-2">
                 {{ person.email }}