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>

dev-docs/API.md

@@ -194,7 +194,9 @@ Persons are sorted: available first, then unavailable (conflict), then already a
   "data": [
     {
       "id": "ulid",
-      "name": "Jan de Vries",
+      "first_name": "Jan",
+      "last_name": "de Vries",
+      "full_name": "Jan de Vries",
       "email": "jan@gmail.com",
       "status": "approved",
       "crowd_type": { "id": "ulid", "name": "Vrijwilliger", "system_type": "VOLUNTEER" },
@@ -204,7 +206,9 @@ Persons are sorted: available first, then unavailable (conflict), then already a
     },
     {
       "id": "ulid",
-      "name": "Ahmed Hassan",
+      "first_name": "Ahmed",
+      "last_name": "Hassan",
+      "full_name": "Ahmed Hassan",
       "email": "ahmed.h@gmail.com",
       "status": "approved",
       "crowd_type": { "id": "ulid", "name": "Vrijwilliger", "system_type": "VOLUNTEER" },
@@ -374,7 +378,7 @@ Response: `{ "confirmed": 2, "errors": [{ "match_id": "ulid3", "error": "User al
 {
   "pending_identity_match": {
     "match_id": "ulid",
-    "matched_user": { "id": "ulid", "name": "Jan", "email": "jan@example.nl" },
+    "matched_user": { "id": "ulid", "first_name": "Jan", "last_name": "", "full_name": "Jan", "email": "jan@example.nl" },
     "matched_on": "email",
     "confidence": "exact"
   }