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/ProjectSetting.php

@@ -6,7 +6,7 @@ use Illuminate\Database\Eloquent\Model;
 
 class ProjectSetting extends Model
 {
-    protected $guarded = [];
+    protected $fillable = [];
 
     public function project()
     {