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/organisation/CompanyDialog.vue

@@ -18,7 +18,8 @@ const isEdit = computed(() => !!props.company)
 const form = ref({
   name: '',
   type: 'supplier' as Company['type'],
-  contact_name: '',
+  contact_first_name: '',
+  contact_last_name: '',
   contact_email: '',
   contact_phone: '',
 })
@@ -44,7 +45,8 @@ watch(() => props.company, (c) => {
     form.value = {
       name: c.name,
       type: c.type,
-      contact_name: c.contact_name ?? '',
+      contact_first_name: c.contact_first_name ?? '',
+      contact_last_name: c.contact_last_name ?? '',
       contact_email: c.contact_email ?? '',
       contact_phone: c.contact_phone ?? '',
     }
@@ -55,7 +57,8 @@ function resetForm() {
   form.value = {
     name: '',
     type: 'supplier',
-    contact_name: '',
+    contact_first_name: '',
+    contact_last_name: '',
     contact_email: '',
     contact_phone: '',
   }
@@ -76,7 +79,8 @@ function onSubmit() {
     const payload = {
       name: form.value.name,
       type: form.value.type,
-      contact_name: form.value.contact_name || null,
+      contact_first_name: form.value.contact_first_name || null,
+      contact_last_name: form.value.contact_last_name || null,
       contact_email: form.value.contact_email || null,
       contact_phone: form.value.contact_phone || null,
     }
@@ -140,11 +144,25 @@ function onSubmit() {
                 :error-messages="errors.type"
               />
             </VCol>
-            <VCol cols="12">
+            <VCol
+              cols="12"
+              md="6"
+            >
               <AppTextField
-                v-model="form.contact_name"
-                label="Contactpersoon"
-                :error-messages="errors.contact_name"
+                v-model="form.contact_first_name"
+                label="Voornaam contactpersoon"
+                :error-messages="errors.contact_first_name"
+                autocomplete="one-time-code"
+              />
+            </VCol>
+            <VCol
+              cols="12"
+              md="6"
+            >
+              <AppTextField
+                v-model="form.contact_last_name"
+                label="Achternaam contactpersoon"
+                :error-messages="errors.contact_last_name"
                 autocomplete="one-time-code"
               />
             </VCol>