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/events/[id]/persons/index.vue

@@ -84,7 +84,7 @@ const statusColor: Record<PersonStatus, string> = {
 }
 
 const headers = [
-  { title: 'Naam', key: 'name' },
+  { title: 'Naam', key: 'full_name' },
   { title: 'E-mail', key: 'email' },
   { title: 'Crowd Type', key: 'crowd_type' },
   { title: 'Status', key: 'status' },
@@ -137,7 +137,7 @@ function onRowClick(_event: Event, row: { item: Person }) {
 function onApprove(person: Person) {
   approvePerson(person.id, {
     onSuccess: () => {
-      successMessage.value = `${person.name} goedgekeurd`
+      successMessage.value = `${person.full_name} goedgekeurd`
       showSuccess.value = true
     },
   })
@@ -155,7 +155,7 @@ function onDeleteConfirm(person: Person) {
 
 function onDeleteExecute() {
   if (!deletingPerson.value) return
-  const name = deletingPerson.value.name
+  const name = deletingPerson.value.full_name
 
   deletePerson(deletingPerson.value.id, {
     onSuccess: () => {
@@ -284,16 +284,16 @@ const crowdTypeOptions = computed(() => [
         hover
         @click:row="onRowClick"
       >
-        <template #item.name="{ item }">
+        <template #item.full_name="{ item }">
           <div class="d-flex align-center gap-x-3">
             <VAvatar
               size="32"
               color="primary"
               variant="tonal"
             >
-              <span class="text-caption">{{ getInitials(item.name) }}</span>
+              <span class="text-caption">{{ getInitials(item.full_name) }}</span>
             </VAvatar>
-            <span>{{ item.name }}</span>
+            <span>{{ item.full_name }}</span>
           </div>
         </template>
 
@@ -387,7 +387,7 @@ const crowdTypeOptions = computed(() => [
     >
       <VCard title="Persoon verwijderen">
         <VCardText>
-          Weet je zeker dat je <strong>{{ deletingPerson?.name }}</strong> wilt verwijderen?
+          Weet je zeker dat je <strong>{{ deletingPerson?.full_name }}</strong> wilt verwijderen?
         </VCardText>
         <VCardActions>
           <VSpacer />