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/ScheduledDatabaseBackupExecution.php

@@ -6,7 +6,17 @@ use Illuminate\Database\Eloquent\Relations\BelongsTo;
 
 class ScheduledDatabaseBackupExecution extends BaseModel
 {
-    protected $guarded = [];
+    protected $fillable = [
+        'status',
+        'message',
+        'size',
+        'filename',
+        'database_name',
+        'finished_at',
+        'local_storage_deleted',
+        's3_storage_deleted',
+        's3_uploaded',
+    ];
 
     protected function casts(): array
     {