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/database/migrations/2026_04_22_100002_add_schema_version_at_open_to_form_submissions.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            // Version the portal rendered at draft-open time. Compared to
+            // schema_version_at_submit by PublicFormSubmissionResource so
+            // the portal can warn about drift before showing the success
+            // screen (S2c D5).
+            $table->unsignedInteger('schema_version_at_open')->nullable()->after('opened_at');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->dropColumn('schema_version_at_open');
+        });
+    }
+};