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/EditPersonDialog.vue

@@ -23,7 +23,8 @@ const { mutate: updatePerson, isPending } = useUpdatePerson(eventIdRef)
 
 const form = ref({
   crowd_type_id: '',
-  name: '',
+  first_name: '',
+  last_name: '',
   email: '',
   phone: '',
   company_id: '',
@@ -39,7 +40,8 @@ const showSuccess = ref(false)
 watch(() => props.person, (p) => {
   form.value = {
     crowd_type_id: p.crowd_type?.id ?? '',
-    name: p.name,
+    first_name: p.first_name,
+    last_name: p.last_name,
     email: p.email,
     phone: p.phone ?? '',
     company_id: p.company?.id ?? '',
@@ -92,7 +94,8 @@ function onSubmit() {
     updatePerson({
       id: props.person.id,
       crowd_type_id: form.value.crowd_type_id,
-      name: form.value.name,
+      first_name: form.value.first_name,
+      last_name: form.value.last_name,
       email: form.value.email,
       phone: form.value.phone || undefined,
       company_id: form.value.company_id || undefined,
@@ -112,7 +115,7 @@ function onSubmit() {
           )
         }
         else if (data?.message) {
-          errors.value = { name: data.message }
+          errors.value = { first_name: data.message }
         }
       },
     })
@@ -141,15 +144,29 @@ function onSubmit() {
                 :error-messages="errors.crowd_type_id"
               />
             </VCol>
-            <VCol cols="12">
+            <VCol
+              cols="12"
+              md="6"
+            >
               <AppTextField
-                v-model="form.name"
-                label="Naam"
+                v-model="form.first_name"
+                label="Voornaam"
                 :rules="[requiredValidator]"
-                :error-messages="errors.name"
+                :error-messages="errors.first_name"
                 autofocus
               />
             </VCol>
+            <VCol
+              cols="12"
+              md="6"
+            >
+              <AppTextField
+                v-model="form.last_name"
+                label="Achternaam"
+                :rules="[requiredValidator]"
+                :error-messages="errors.last_name"
+              />
+            </VCol>
             <VCol cols="12">
               <AppTextField
                 v-model="form.email"