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/SharedEnvironmentVariable.php

@@ -22,6 +22,9 @@ class SharedEnvironmentVariable extends Model
         'is_multiline',
         'is_literal',
         'is_shown_once',
+
+        // Metadata
+        'version',
     ];
 
     protected $casts = [