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/layouts/components/UserProfile.vue

@@ -10,7 +10,7 @@ const { mutate: logout, isPending: isLoggingOut } = useLogout()
 const user = computed(() => authStore.user)
 
 const initials = computed(() => {
-  const name = user.value?.name?.trim() ?? ''
+  const name = user.value?.full_name?.trim() ?? ''
   if (!name)
     return '?'
   const parts = name.split(/\s+/).filter(Boolean)
@@ -48,7 +48,7 @@ function handleLogout() {
     <VImg
       v-if="avatarSrc"
       :src="avatarSrc"
-      :alt="user?.name ?? ''"
+      :alt="user?.full_name ?? ''"
       cover
     />
     <span
@@ -67,7 +67,7 @@ function handleLogout() {
       <VList>
         <VListItem class="text-high-emphasis">
           <VListItemTitle class="font-weight-semibold">
-            {{ user?.name ?? '—' }}
+            {{ user?.full_name ?? '—' }}
           </VListItemTitle>
           <VListItemSubtitle class="text-wrap">
             {{ user?.email ?? '' }}