feat(#112): email accusé resoumission aux parents

Après PATCH reprise-resoumettre (parent), envoi à chaque parent du
dossier : confirmation resoumission + rappel numero_dossier.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/modules/mail/mail.service.spec.ts

@@ -98,4 +98,21 @@ describe('MailService (ticket #28)', () => {
     expect(html).toContain(`https://app.test/reset-password?token=${encodeURIComponent(token)}`);
     expect(html).not.toContain('create-password');
   });
+
+  it('sendResoumissionPendingEmail — accusé resoumission avec n° dossier', async () => {
+    await service.sendResoumissionPendingEmail(
+      'parent@test.fr',
+      'Claire',
+      'MARTIN',
+      '2026-000024',
+    );
+    expect(sendEmailSpy).toHaveBeenCalledTimes(1);
+    const [to, subject, html] = sendEmailSpy.mock.calls[0];
+    expect(to).toBe('parent@test.fr');
+    expect(subject).toContain('resoumis');
+    expect(subject).toContain('2026-000024');
+    expect(html).toContain('Claire');
+    expect(html).toContain('2026-000024');
+    expect(html).toContain('en attente de validation');
+  });
 });