feat(form-builder): add failure_response_code column to form_submissions

Per RFC-WS-6 §Q3 v1.3 addition 2 + ARCH-BINDINGS §7.1 v1.2.

Denormalised mirror of the FormBindingApplicatorException subclass
classification, written by ApplyBindingsOnFormSubmit's outer-transaction
catch block (D2) when apply_status='failed'. Drives response-shape copy.
NULL when apply_status is not 'failed'.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

api/database/migrations/2026_05_08_000001_add_failure_response_code_to_form_submissions.php

@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * Per RFC-WS-6 §Q3 v1.3 addition 2 + ARCH-BINDINGS §7.1 v1.2.
+ *
+ * Mirrors the FormBindingApplicatorException subclass classification onto
+ * a denormalised string column for response-shape rendering. Values:
+ *   - schema_config_error  (HTTP 422)
+ *   - temporary_error      (HTTP 503)
+ *   - data_integrity_error (HTTP 422)
+ *   - unknown_error        (HTTP 500)
+ *
+ * Read by the response renderer when apply_status='failed' to surface
+ * contextual user-facing error copy. The exception subclass on the
+ * action-failures row is the canonical source; this column is the
+ * response-shape driver. Both reference the same submission.id ULID.
+ *
+ * NULL when apply_status is not 'failed' (per the lifecycle:
+ * pending/completed/partial → no failure to classify).
+ */
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->string('failure_response_code', 40)
+                ->nullable()
+                ->after('apply_status');
+
+            $table->index('failure_response_code', 'fs_failure_response_code_idx');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('form_submissions', function (Blueprint $table): void {
+            $table->dropIndex('fs_failure_response_code_idx');
+            $table->dropColumn('failure_response_code');
+        });
+    }
+};