Function process_uploads 

pub async fn process_uploads(
    db: &dyn SyncBookkeeping,
    cloud_home: &dyn CloudHome,
    encryption: &RwLock<EncryptionService>,
    library_dir: &Path,
    observer: Option<&dyn BlobUploadObserver>,
) -> Result<usize, String>

Process pending uploads: read local file, encrypt, write to cloud.

Returns the number of successful uploads. Stops at the first failure so we don’t push out-of-order. After each successful upload, observer (if any) is notified so the host can run its own bookkeeping.