Function multipart_scan 

pub async fn multipart_scan(
    profile_id: ProfileId,
    bucket: BucketId,
    older_than_secs: Option<u64>,
    store: State<'_, ProfileStoreHandle>,
    pool: State<'_, S3ClientPoolHandle>,
    multipart_table: State<'_, MultipartTableHandle>,
) -> Result<Vec<MultipartUpload>, AppError>

List all in-progress multipart uploads for bucket, classifying each as Brows3r (in our multipart_active table) or Unknown (foreign).

Optionally filter out uploads younger than older_than_secs seconds.

Satisfies AC-4 cleanup scanner requirement.