feat(form-builder): schema drift detection + PUT auto_save_count

S2c D5 completion: schema_version_at_open column + drift semantics.

- Migration 2026_04_22_100002 adds unsignedInteger schema_version_at_open.
  Recorded by FormSubmissionService::createDraft at the moment the
  portal first renders the form.
- PublicFormSubmissionResource.schema_drift now compares
  schema_version_at_open vs schema_version_at_submit (or
  schema.version for active drafts) so organiser edits during an
  open draft surface as drift on subsequent PUT/submit responses.
- PublicFormSubmissionController::update routes through
  FormSubmissionService::saveDraft so auto_save_count increments
  and the FormSubmissionDraftUpdated event fires per PUT.
- bootstrap/app.php: FormRequest ValidationException on
  /api/v1/public/forms/* is now re-wrapped into the D6 envelope with
  code=VALIDATION_FAILED, so public endpoints emit one consistent
  error shape regardless of layer.

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

api/app/Services/FormBuilder/FormSubmissionService.php

@@ -61,6 +61,7 @@ final class FormSubmissionService
             $submission->status = FormSubmissionStatus::DRAFT->value;
             $submission->is_test = (bool) ($context['is_test'] ?? false);
             $submission->opened_at = $context['opened_at'] ?? now();
+            $submission->schema_version_at_open = (int) $schema->version;
             $submission->idempotency_key = $context['idempotency_key'] ?? null;
             $submission->public_submitter_name = $context['public_submitter_name'] ?? null;
             $submission->public_submitter_email = $context['public_submitter_email'] ?? null;