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/portal/src/pages/register/[eventSlug].vue

@@ -37,7 +37,8 @@ const submitError = ref<string | null>(null)
 const { errors, defineField, validateField, setFieldValue } = useForm({
   validationSchema: toTypedSchema(fullRegistrationSchema),
   initialValues: {
-    name: '',
+    first_name: '',
+    last_name: '',
     email: '',
     phone: '',
     tshirt_size: '',
@@ -50,7 +51,8 @@ const { errors, defineField, validateField, setFieldValue } = useForm({
   },
 })
 
-const [name] = defineField('name')
+const [firstName] = defineField('first_name')
+const [lastName] = defineField('last_name')
 const [email] = defineField('email')
 const [phone] = defineField('phone')
 const [tshirtSize] = defineField('tshirt_size')
@@ -64,7 +66,8 @@ const [motivationOther] = defineField('motivation_other')
 // Pre-fill authenticated user data
 watch(() => authStore.user, user => {
   if (user) {
-    setFieldValue('name', user.name)
+    setFieldValue('first_name', user.first_name)
+    setFieldValue('last_name', user.last_name)
     setFieldValue('email', user.email)
   }
 }, { immediate: true })
@@ -154,10 +157,10 @@ const formattedDates = computed(() => {
 })
 
 // Step field mapping for validation (0-based)
-type FormField = 'name' | 'email' | 'phone' | 'tshirt_size' | 'first_aid' | 'allergies' | 'access_requirements' | 'driving_licence' | 'motivation' | 'motivation_other'
+type FormField = 'first_name' | 'last_name' | 'email' | 'phone' | 'tshirt_size' | 'first_aid' | 'allergies' | 'access_requirements' | 'driving_licence' | 'motivation' | 'motivation_other'
 
 const stepFields: Record<number, FormField[]> = {
-  0: ['name', 'email', 'phone'],
+  0: ['first_name', 'last_name', 'email', 'phone'],
   1: ['tshirt_size', 'first_aid', 'allergies', 'access_requirements', 'driving_licence'],
   2: ['motivation', 'motivation_other'],
 }
@@ -265,7 +268,8 @@ async function onSubmit() {
   }))
 
   const payload: VolunteerRegistrationForm = {
-    name: name.value ?? '',
+    first_name: firstName.value ?? '',
+    last_name: lastName.value ?? '',
     email: email.value ?? '',
     phone: phone.value ?? '',
     tshirt_size: tshirtSize.value ?? '',
@@ -452,7 +456,7 @@ async function onSubmit() {
         variant="tonal"
         class="mb-4"
       >
-        Je bent ingelogd als {{ authStore.user?.name }}. Je gegevens zijn automatisch ingevuld.
+        Je bent ingelogd als {{ authStore.user?.full_name }}. Je gegevens zijn automatisch ingevuld.
       </VAlert>
 
       <div
@@ -624,10 +628,24 @@ async function onSubmit() {
                     md="6"
                   >
                     <VTextField
-                      v-model="name"
-                      label="Naam *"
-                      placeholder="Je volledige naam"
-                      :error-messages="errors.name"
+                      v-model="firstName"
+                      label="Voornaam *"
+                      placeholder="Je voornaam"
+                      :error-messages="errors.first_name"
+                      :disabled="authStore.isAuthenticated"
+                      density="comfortable"
+                    />
+                  </VCol>
+
+                  <VCol
+                    cols="12"
+                    md="6"
+                  >
+                    <VTextField
+                      v-model="lastName"
+                      label="Achternaam *"
+                      placeholder="Je achternaam"
+                      :error-messages="errors.last_name"
                       :disabled="authStore.isAuthenticated"
                       density="comfortable"
                     />

apps/portal/src/schemas/registrationSchema.ts

@@ -1,7 +1,8 @@
 import { z } from 'zod'
 
 export const step1Schema = z.object({
-  name: z.string().min(1, 'Naam is verplicht').max(255),
+  first_name: z.string().min(1, 'Voornaam is verplicht').max(255),
+  last_name: z.string().min(1, 'Achternaam is verplicht').max(255),
   email: z.string().min(1, 'E-mailadres is verplicht').email('Ongeldig e-mailadres').max(255),
   phone: z.string().max(50).optional().or(z.literal('')),
 })

apps/portal/src/stores/useAuthStore.ts

@@ -4,7 +4,9 @@ import { apiClient } from '@/lib/axios'
 
 interface PortalUser {
   id: string
-  name: string
+  first_name: string
+  last_name: string
+  full_name: string
   email: string
 }
 

apps/portal/src/types/registration.ts

@@ -42,7 +42,8 @@ export interface VolunteerAvailability {
 
 export interface VolunteerRegistrationForm {
   // Step 1
-  name: string
+  first_name: string
+  last_name: string
   email: string
   phone: string
   // Step 2