feat(form-builder): FormSubmission cast + factory state for failure_response_code

Per RFC-WS-6 §Q3 v1.3 addition 2.

- Added 'failure_response_code' to FormSubmission $fillable + 'string' cast.
  Plain string (not enum) — the exception subclass on
  form_submission_action_failures is the canonical classification source;
  this column is a denormalised mirror for response-shape rendering.
- Factory fluent state method withFailureResponseCode() with documentation
  of the four valid values.

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

api/app/Models/FormBuilder/FormSubmission.php

@@ -11,9 +11,9 @@ use App\Models\Event;
 use App\Models\Organisation;
 use App\Models\Scopes\OrganisationScope;
 use App\Models\User;
+use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Concerns\HasUlids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
-use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
@@ -32,7 +32,7 @@ final class FormSubmission extends Model
 
     protected static function booted(): void
     {
-        static::addGlobalScope(new OrganisationScope());
+        self::addGlobalScope(new OrganisationScope);
     }
 
     /** @return array{column: string} */
@@ -72,6 +72,7 @@ final class FormSubmission extends Model
         'first_interacted_at',
         'idempotency_key',
         'identity_match_status',
+        'failure_response_code',
     ];
 
     /** @var array<string, string> */
@@ -80,6 +81,11 @@ final class FormSubmission extends Model
         'review_status' => FormSubmissionReviewStatus::class,
         'apply_status' => ApplyStatus::class,
         'apply_completed_at' => 'datetime',
+        // Plain string (not enum). The exception subclass on
+        // form_submission_action_failures is the canonical source of
+        // classification truth; this column is a denormalised mirror for
+        // response-shape rendering. Per RFC-WS-6 §Q3 v1.3 addition 2.
+        'failure_response_code' => 'string',
         'schema_snapshot' => 'array',
         'is_test' => 'bool',
         'submitted_at' => 'datetime',