fix(backups): validate S3 storage before backup scheduling

Prevent scheduled database backups from enabling S3 uploads without a valid team-owned storage configuration, and preserve the previous S3 storage ID in missing-storage error messages.

Add coverage for backup edit/create validation and S3 upload failure messaging.

app/Models/ScheduledDatabaseBackupExecution.php

@@ -23,6 +23,7 @@ class ScheduledDatabaseBackupExecution extends BaseModel
     protected function casts(): array
     {
         return [
+            'size' => 'integer',
             's3_uploaded' => 'boolean',
             'local_storage_deleted' => 'boolean',
             's3_storage_deleted' => 'boolean',