feat(form-builder): detect duplicate submissions by email on same form schema

Informational hint on the confirmation page when the same email has
already submitted the form. Not a block — the submission proceeds
normally. Privacy-safe: only shown to the submitter themselves.

Scope: same form_schema_id only. Cross-form/cross-event detection
would leak info about other forms.

- New FormSubmissionDuplicateDetector service queries by
  form_submissions.public_submitter_email (trim + case-insensitive)
  scoped to the schema, status=submitted, excluding the current
  submission. Errors are swallowed + logged so a detector failure
  never blocks the submit response.
- PublicFormSubmissionController enriches the submit response by
  setting a transient duplicate_submission_data attribute on the
  submission before resource serialisation.
- PublicFormSubmissionResource serialises a duplicate_submission
  block with count, first_submitted_at, plus backend-authored
  Dutch title + body (plural-agreement + IntlDateFormatter for
  "23 april 2026"-style long-form dates). Null when no priors,
  no email, or detector error.
- DuplicateSubmissionHint.vue (warning-typed tonal VAlert) above
  IdentityMatchBanner on FormConfirmation. Prefers backend copy
  with Intl-based Dutch date fallback for safety.
- 16 new backend assertions across the detector and the full
  submit-response flow; 5 new Vitest assertions for the hint.

Note on scope: spec suggested extracting email from values via
schema binding; the codebase's public flow captures submitter
email in a guaranteed column (public_submitter_email) populated
by the stepper's Contactgegevens step. Using that directly is
both simpler and more correct for the duplicate-by-submitter
semantic. When FORM-05's binding-based extractor lands, this
detector can migrate without changing its public API.

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

apps/portal/components.d.ts

@@ -33,6 +33,7 @@ declare module 'vue' {
     CustomRadiosWithImage: typeof import('./src/@core/components/app-form-elements/CustomRadiosWithImage.vue')['default']
     DialogCloseBtn: typeof import('./src/@core/components/DialogCloseBtn.vue')['default']
     DropZone: typeof import('./src/@core/components/DropZone.vue')['default']
+    DuplicateSubmissionHint: typeof import('./src/components/public-form/DuplicateSubmissionHint.vue')['default']
     EventCard: typeof import('./src/components/portal/EventCard.vue')['default']
     FieldAvailabilityPicker: typeof import('./src/components/public-form/FieldAvailabilityPicker.vue')['default']
     FieldBoolean: typeof import('./src/components/public-form/FieldBoolean.vue')['default']