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/sections/AssignShiftDialog.vue

@@ -25,6 +25,11 @@ const selectedPersonId = ref<string>('')
 const errors = ref<Record<string, string>>({})
 const showSuccess = ref(false)
 
+const generalError = computed(() => {
+  const keys = Object.keys(errors.value).filter(k => k !== 'person_id')
+  return keys.length ? errors.value[keys[0]] : ''
+})
+
 // Check for overlap warning
 const hasOverlapWarning = computed(() => {
   if (!selectedPersonId.value || !props.shift) return false
@@ -34,7 +39,7 @@ const hasOverlapWarning = computed(() => {
 
 const personItems = computed(() =>
   persons.value.map((p: Person) => ({
-    title: `${p.name} — ${p.email}`,
+    title: `${p.full_name} — ${p.email}`,
     value: p.id,
     props: {
       subtitle: p.crowd_type?.name ?? '',
@@ -106,6 +111,15 @@ function onSubmit() {
 
         <VDivider class="mb-4" />
 
+        <VAlert
+          v-if="generalError"
+          type="error"
+          variant="tonal"
+          class="mb-4"
+        >
+          {{ generalError }}
+        </VAlert>
+
         <!-- Person search -->
         <VAutocomplete
           v-model="selectedPersonId"