docs(form-builder): document S3a PR 2 complex field types and update FORM-05 stub note

- Add VitePress pages for AVAILABILITY_PICKER and SECTION_PRIORITY
  and a TAG_PICKER configuration note. Wire them into the organisator
  sidebar under a new Formulieren section alongside the existing
  "Wat is een formulier" page.
- BACKLOG.md: nuance FORM-05 — the stub-shaped behaviour for public
  event_registration submissions is already shipping via the existing
  TriggerPersonIdentityMatchOnFormSubmit listener (writes 'pending').
  The real work (PersonIdentityService::detectMatchesByValues + an
  extra branch in resolveStatus) is what remains. Added a done entry
  for S3a PR 2 to the Opgeloste items list.
- API.md: add VALIDATION_FAILED to the public-form error code table
  and document the SECTION_PRIORITY shape error messages (Dutch copy
  served under errors."values.{slug}").
- COPY_CATALOGUE.md: new S3a PR 2 section capturing the seeder
  help_text, the IdentityMatchBanner copy (clearly marking the
  backend message as authoritative), all empty/error state copy for
  the three new components, and the SECTION_PRIORITY shape error
  strings.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dev-docs/BACKLOG.md

@@ -341,21 +341,28 @@ shifts claimen zonder toegang tot de Organizer app.
 
 ### FORM-05 — Smart identity-match on public submission values
 
-Public form submissions (subject_type=null) currently always get
-identity_match_status='pending' because the listener needs a Person
-to match against and public submissions don't create one yet.
+**Stub-status (S3a PR 2, 2026-04-23):** Public event_registration
+submissions landen al met `identity_match_status='pending'` via de
+bestaande `TriggerPersonIdentityMatchOnFormSubmit` listener. De portal
+`IdentityMatchBanner` leest dit veld en toont de juiste copy. Contract
+ligt vast in `tests/Feature/FormBuilder/Listeners/TriggerPersonIdentityMatchOnFormSubmitTest`.
 
-Improve: when a public event_registration submission arrives, extract
-email + first_name + last_name from the submission values (via the
-schema's binding config) and call a new PersonIdentityService method:
-    detectMatchesByValues(array $values, string $organisationId): MatchResult
+**Resterend werk (de eigenlijke FORM-05):** public form submissions
+(subject_type=null) krijgen momenteel *altijd* 'pending' omdat er nog
+geen Person bestaat om tegen te matchen. Breid uit met:
 
-Set identity_match_status to matched / pending / none based on actual
-lookup. This gives the portal-form UX a meaningful signal instead of
-a constant pending.
+- Nieuwe methode op PersonIdentityService:
+    `detectMatchesByValues(array $values, string $organisationId): MatchResult`
+- Een extra tak in `TriggerPersonIdentityMatchOnFormSubmit::resolveStatus`
+  die voor public submissions de values uit `FormSubmission->values`
+  extraheert (email / first_name / last_name via de schema binding),
+  deze methode aanroept, en 'matched' / 'pending' / 'none' schrijft.
 
-Priority: Medium. Can bundle with organizer person_identity_matches UI
-(which is also still a frontend gap).
+Zo krijgt de portal-UX een betekenisvol signaal in plaats van een
+constante 'pending'.
+
+Prioriteit: Medium. Kan gebundeld worden met de organizer
+`person_identity_matches` UI (ook nog een frontend gap).
 
 ---
 
@@ -491,6 +498,7 @@ De volgende items zijn geïmplementeerd en afgerond (673+ tests):
 - ~~Registration settings (show_in_registration)~~ ✅
 - ~~Premium portal wizard (banner, branding, success page)~~ ✅
 - ~~Global error handling (useNotificationStore + axios 422 interceptor)~~ ✅
+- ~~S3a PR 2: TAG_PICKER / AVAILABILITY_PICKER / SECTION_PRIORITY renderen in het publieke registratieformulier. Seeder uitgebreid met twee showcase-velden + parent-level VOLUNTEER time slot + duplicate section name voor dedup-dekking. SECTION_PRIORITY waarde-shape gevalideerd in FormValueService. `FormSubmissionResource` krijgt admin-facing `identity_match` block. 64 nieuwe assertions over backend + Vitest.~~ ✅
 
 ---