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();
         }

api/app/Http/Resources/FormBuilder/PublicFormSubmissionResource.php

@@ -69,21 +69,30 @@ final class PublicFormSubmissionResource extends JsonResource
 
     private function computeSchemaDrift(): bool
     {
-        if ($this->schema_version_at_submit === null) {
-            // Draft phase: drift is "version advanced since the submission
-            // was opened". We use schema.version vs the version the portal
-            // saw via PublicFormSchemaResource at open time — which is
-            // only derivable after submit. For drafts, schema_drift stays
-            // false; the submit response is the authoritative check.
+        // The draft was opened against version_at_open; at submit time the
+        // schema's current version is frozen into version_at_submit. Drift
+        // means the organiser edited the schema between the portal loading
+        // the form and the user hitting submit.
+        //
+        // Drafts without a submit stamp keep the comparison live against
+        // the current schema.version so an organiser edit during an open
+        // draft also surfaces drift on subsequent PUT/GET calls.
+
+        $atOpen = $this->schema_version_at_open;
+        if ($atOpen === null) {
             return false;
         }
 
-        $schema = $this->schema;
-        if ($schema === null) {
-            return false;
+        $other = $this->schema_version_at_submit;
+        if ($other === null) {
+            $schema = $this->schema;
+            if ($schema === null) {
+                return false;
+            }
+            $other = (int) $schema->version;
         }
 
-        return (int) $schema->version !== (int) $this->schema_version_at_submit;
+        return (int) $atOpen !== (int) $other;
     }
 
     /**

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',

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;