feat(form-builder): extend public form backend for S3a PR 2

- Seed AVAILABILITY_PICKER and SECTION_PRIORITY demo fields in the
  event_registration showcase, and augment seedEchtFeesten with a
  parent-level VOLUNTEER time slot pair + a standard registration-
  visible section whose name duplicates a child section so the
  PublicFormController dedup path is exercised end-to-end.
- Validate SECTION_PRIORITY value shape in FormValueService: arrays of
  { section_id, priority } with unique section_ids + priorities in 1..5,
  max 5 entries, and section_ids scoped to the schema's event tree
  (parent + children). Error envelope is the standard VALIDATION_FAILED
  FieldValidationException shape so the portal renders errors next to
  the field.
- Enrich admin-facing FormSubmissionResource with a nested identity_match
  block mirroring the PublicFormSubmissionResource contract (status only;
  leaves room for future matched_user_id / confidence).
- Lock in the FORM-05 stub contract with 6 tests against the existing
  TriggerPersonIdentityMatchOnFormSubmit listener (no new listener was
  needed — the current one already writes 'pending' for public
  event_registration submissions per ARCH §31.1).
- 24 new backend assertions across seeder, shape validation, listener
  state matrix, and resource serialisation.

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

api/database/seeders/FormBuilderDevSeeder.php

@@ -293,6 +293,29 @@ final class FormBuilderDevSeeder
                 'display_width' => 'full',
                 'value_storage_hint' => FormValueStorageHint::JSON,
             ],
+            [
+                'type' => FormFieldType::AVAILABILITY_PICKER,
+                'slug' => 'beschikbaarheid',
+                'label' => 'Wanneer ben je beschikbaar?',
+                'help_text' => 'Vink alle dagdelen aan waarop je kunt werken.',
+                'is_required' => true,
+                'is_filterable' => false,
+                'display_width' => 'full',
+                'value_storage_hint' => FormValueStorageHint::JSON,
+            ],
+            [
+                'type' => FormFieldType::SECTION_PRIORITY,
+                'slug' => 'sectie_voorkeur',
+                'label' => 'Bij welke sectie wil je het liefst werken?',
+                'help_text' => 'Sleep je voorkeuren in volgorde. Nummer 1 is je eerste keuze.',
+                // UI soft cap; the hard cap of 5 lives in
+                // FormValueService shape validation.
+                'validation_rules' => ['max_priorities' => 3],
+                'is_required' => false,
+                'is_filterable' => false,
+                'display_width' => 'full',
+                'value_storage_hint' => FormValueStorageHint::JSON,
+            ],
             [
                 'type' => FormFieldType::TEXTAREA,
                 'slug' => 'opmerkingen',
@@ -340,6 +363,7 @@ final class FormBuilderDevSeeder
                 'field_type' => $def['type']->value,
                 'slug' => $def['slug'],
                 'label' => $def['label'],
+                'help_text' => $def['help_text'] ?? null,
                 'options' => $def['options'] ?? null,
                 'validation_rules' => $def['validation_rules'] ?? null,
                 'is_required' => $def['is_required'] ?? false,