feat(form-builder): add FormSubmissionActionFailure model + apply_status casts (WS-6)

- FormSubmissionActionFailure: audit model, no organisation_id (FK-chain
  tenancy per RFC V3), open/resolved/dismissed scopes, canBeRetried()
  helper. Morph alias 'form_submission_action_failure' registered for
  future activity-log subject references.
- FormSubmission: apply_status (ApplyStatus enum cast),
  apply_completed_at (datetime), actionFailures() HasMany,
  scopePendingApply().

Refs: RFC-WS-6.md §3 (Q5), §4 (V3)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/Models/FormBuilder/FormSubmission.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace App\Models\FormBuilder;
 
+use App\Enums\FormBuilder\ApplyStatus;
 use App\Enums\FormBuilder\FormSubmissionReviewStatus;
 use App\Enums\FormBuilder\FormSubmissionStatus;
 use App\Models\Event;
@@ -12,6 +13,7 @@ use App\Models\Scopes\OrganisationScope;
 use App\Models\User;
 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;
@@ -76,6 +78,8 @@ final class FormSubmission extends Model
     protected $casts = [
         'status' => FormSubmissionStatus::class,
         'review_status' => FormSubmissionReviewStatus::class,
+        'apply_status' => ApplyStatus::class,
+        'apply_completed_at' => 'datetime',
         'schema_snapshot' => 'array',
         'is_test' => 'bool',
         'submitted_at' => 'datetime',
@@ -134,4 +138,22 @@ final class FormSubmission extends Model
     {
         return $this->hasMany(FormSubmissionDelegation::class);
     }
+
+    /** @return HasMany<FormSubmissionActionFailure, $this> */
+    public function actionFailures(): HasMany
+    {
+        return $this->hasMany(FormSubmissionActionFailure::class);
+    }
+
+    /**
+     * RFC-WS-6 §3 (Q4) — submissions awaiting an applicator pass. Excludes
+     * NULL apply_status legacy rows by design (RFC O1).
+     *
+     * @param  Builder<FormSubmission>  $query
+     * @return Builder<FormSubmission>
+     */
+    protected function scopePendingApply(Builder $query): Builder
+    {
+        return $query->where('apply_status', ApplyStatus::PENDING->value);
+    }
 }