refactor: define explicit fillable attributes on all Eloquent models

Replace $guarded usage with explicit $fillable arrays across all models.
Sync fillable definitions with current database schema and add tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Models/LocalPersistentVolume.php

@@ -7,7 +7,15 @@ use Symfony\Component\Yaml\Yaml;
 
 class LocalPersistentVolume extends BaseModel
 {
-    protected $guarded = [];
+    protected $fillable = [
+        'name',
+        'mount_path',
+        'host_path',
+        'container_id',
+        'resource_type',
+        'resource_id',
+        'is_preview_suffix_enabled',
+    ];
 
     protected $casts = [
         'is_preview_suffix_enabled' => 'boolean',