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/Models/FormBuilder/FormSubmission.php

@@ -40,6 +40,7 @@ final class FormSubmission extends Model
         'reviewed_at',
         'review_notes',
         'submitted_at',
+        'schema_version_at_open',
         'schema_version_at_submit',
         'schema_snapshot',
         'submission_duration_seconds',
@@ -65,6 +66,7 @@ final class FormSubmission extends Model
         'opened_at' => 'datetime',
         'first_interacted_at' => 'datetime',
         'public_submitter_ip_anonymised_at' => 'datetime',
+        'schema_version_at_open' => 'int',
         'schema_version_at_submit' => 'int',
         'submission_duration_seconds' => 'int',
         'auto_save_count' => 'int',