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/Http/Controllers/Api/V1/FormBuilder/PublicFormSubmissionController.php

@@ -86,12 +86,13 @@ final class PublicFormSubmissionController extends Controller
 
         $values = (array) ($request->validated('values') ?? []);
         if ($values !== []) {
-            // Service throws FieldValidationException (422) on validation
-            // failure; the handler renders the standardised envelope.
-            $this->valueService->upsertMany($submission, $values, null);
+            // saveDraft → upsertMany + auto_save_count++ + FormSubmissionDraftUpdated
+            // event. FieldValidationException bubbles to the D6 envelope.
+            $this->submissionService->saveDraft($submission, $values, null);
         }
 
         if ($request->filled('first_interacted_at')) {
+            $submission->refresh();
             $submission->first_interacted_at ??= $request->validated('first_interacted_at');
             $submission->save();
         }