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>

api/app/Http/Resources/FormBuilder/PublicFormSubmissionResource.php

@@ -47,6 +47,7 @@ final class PublicFormSubmissionResource extends JsonResource
 
         $schemaDrift = $this->computeSchemaDrift();
         $identityMatch = $this->formatIdentityMatch();
+        $duplicateSubmission = $this->formatDuplicateSubmission();
 
         return [
             'id' => $this->id,
@@ -58,6 +59,7 @@ final class PublicFormSubmissionResource extends JsonResource
             'schema_drift' => $schemaDrift,
             'values' => $values,
             'identity_match' => $identityMatch,
+            'duplicate_submission' => $duplicateSubmission,
             'opened_at' => optional($this->opened_at)->toIso8601String(),
             'first_interacted_at' => optional($this->first_interacted_at)->toIso8601String(),
             'submitted_at' => optional($this->submitted_at)->toIso8601String(),
@@ -95,6 +97,74 @@ final class PublicFormSubmissionResource extends JsonResource
         return (int) $atOpen !== (int) $other;
     }
 
+    /**
+     * Duplicate-submission signal. The controller sets
+     * `duplicate_submission_data` transiently on the submission model
+     * after calling FormSubmissionDuplicateDetector::formatForResponse;
+     * when null (no priors, missing email, or detector error) the
+     * response renders null and the portal skips the hint.
+     *
+     * Copy source of truth: frontend falls back to Dutch strings but
+     * the backend attaches `title` + `body` so the portal can render
+     * without maintaining its own plural-agreement logic.
+     *
+     * @return array<string, mixed>|null
+     */
+    private function formatDuplicateSubmission(): ?array
+    {
+        $raw = $this->getAttribute('duplicate_submission_data');
+        if (! is_array($raw)) {
+            return null;
+        }
+
+        $count = (int) ($raw['count'] ?? 0);
+        if ($count < 1) {
+            return null;
+        }
+
+        $firstIso = (string) ($raw['first_submitted_at'] ?? '');
+        $formattedDate = $firstIso !== ''
+            ? $this->formatDutchLongDate($firstIso)
+            : '';
+
+        return [
+            'count' => $count,
+            'first_submitted_at' => $firstIso,
+            'title' => 'Je hebt je eerder al aangemeld',
+            'body' => $count === 1
+                ? sprintf(
+                    'Op %s heb je dit formulier ook al ingevuld. De organisator ziet beide aanmeldingen en neemt zo snel mogelijk contact op.',
+                    $formattedDate,
+                )
+                : sprintf(
+                    'Je hebt dit formulier al %d keer eerder ingevuld (voor het eerst op %s). De organisator ziet alle aanmeldingen en neemt zo snel mogelijk contact op.',
+                    $count,
+                    $formattedDate,
+                ),
+        ];
+    }
+
+    private function formatDutchLongDate(string $iso): string
+    {
+        try {
+            $formatter = new \IntlDateFormatter(
+                'nl_NL',
+                \IntlDateFormatter::LONG,
+                \IntlDateFormatter::NONE,
+                'Europe/Amsterdam',
+            );
+            $ts = strtotime($iso);
+            if ($ts === false) {
+                return '';
+            }
+            $out = $formatter->format($ts);
+
+            return is_string($out) ? $out : '';
+        } catch (\Throwable) {
+            return '';
+        }
+    }
+
     /**
      * Identity-match signal per ARCH §31.1. Populated by the
      * TriggerPersonIdentityMatchOnFormSubmit listener on