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/tests/unit/DuplicateSubmissionHint.spec.ts

@@ -0,0 +1,80 @@
+import { mount } from '@vue/test-utils'
+import { describe, expect, it } from 'vitest'
+import DuplicateSubmissionHint from '@/components/public-form/DuplicateSubmissionHint.vue'
+import type { PublicFormSubmissionDuplicate } from '@/types/formBuilder'
+
+function mountHint(data: PublicFormSubmissionDuplicate | null) {
+  return mount(DuplicateSubmissionHint, {
+    props: { data },
+    global: {
+      stubs: {
+        VAlert: {
+          name: 'VAlert',
+          props: ['type', 'variant', 'prominent'],
+          template: '<div class="v-alert-stub" :data-type="type" :data-variant="variant"><slot/></div>',
+        },
+      },
+    },
+  })
+}
+
+describe('DuplicateSubmissionHint', () => {
+  it('renders nothing when data is null', () => {
+    const w = mountHint(null)
+
+    expect(w.find('.v-alert-stub').exists()).toBe(false)
+  })
+
+  it('prefers the backend title and body when provided', () => {
+    const w = mountHint({
+      count: 1,
+      first_submitted_at: '2026-04-22T10:00:00+00:00',
+      title: 'Je hebt je eerder al aangemeld',
+      body: 'Op 22 april 2026 heb je dit formulier ook al ingevuld.',
+    })
+
+    expect(w.text()).toContain('Je hebt je eerder al aangemeld')
+    expect(w.text()).toContain('Op 22 april 2026')
+  })
+
+  it('falls back to singular copy when the backend body is missing (count=1)', () => {
+    const w = mountHint({
+      count: 1,
+      first_submitted_at: '2026-04-22T10:00:00+00:00',
+      title: '',
+      body: '',
+    })
+
+    // Fallback title + body.
+    expect(w.text()).toContain('Je hebt je eerder al aangemeld')
+    expect(w.text()).toMatch(/Op\s+22\s+april\s+2026.*ook al ingevuld/)
+    expect(w.text()).toContain('De organisator ziet beide aanmeldingen')
+  })
+
+  it('falls back to plural copy with count when the backend body is missing', () => {
+    const w = mountHint({
+      count: 3,
+      first_submitted_at: '2026-04-22T10:00:00+00:00',
+      title: '',
+      body: '',
+    })
+
+    expect(w.text()).toContain('3 keer eerder ingevuld')
+    expect(w.text()).toContain('22 april 2026')
+    expect(w.text()).toContain('De organisator ziet alle aanmeldingen')
+  })
+
+  it('renders as a warning-typed tonal VAlert', () => {
+    const w = mountHint({
+      count: 1,
+      first_submitted_at: '2026-04-22T10:00:00+00:00',
+      title: 'x',
+      body: 'y',
+    })
+
+    const alert = w.find('.v-alert-stub')
+    expect(alert.exists()).toBe(true)
+    expect(alert.attributes('data-type')).toBe('warning')
+    expect(alert.attributes('data-variant')).toBe('tonal')
+  })
+})