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/GitlabApp.php

@@ -4,6 +4,24 @@ namespace App\Models;
 
 class GitlabApp extends BaseModel
 {
+    protected $fillable = [
+        'name',
+        'organization',
+        'api_url',
+        'html_url',
+        'custom_port',
+        'custom_user',
+        'is_system_wide',
+        'is_public',
+        'app_id',
+        'app_secret',
+        'oauth_id',
+        'group_name',
+        'public_key',
+        'webhook_token',
+        'deploy_key_id',
+    ];
+
     protected $hidden = [
         'webhook_token',
         'app_secret',