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/S3Storage.php

@@ -15,6 +15,7 @@ class S3Storage extends BaseModel
     use HasFactory, HasSafeStringAttribute;
 
     protected $fillable = [
+        'team_id',
         'name',
         'description',
         'region',