fix(fuse): give uploads under a low cap the time their share needs

The upload deadline assumed at least 32 KiB/s. Under an upload cap shared
by every drain worker and sync task, a large file ran out of time on every
attempt, took the mount offline and restarted from byte 0. The deadline
now counts the upload's share of the cap as the slowest rate when that
share is lower (B231).

crates/pdfs-fuse/src/sync/engine.rs

@@ -22,7 +22,7 @@ const POLL_INTERVAL: Duration = Duration::from_secs(120);
 /// per-item work is a network round-trip, so a folder with thousands of files is
 /// unusable done one-at-a-time; this bounds concurrency like the bulk-upload
 /// engine does.
-pub(super) const SYNC_CONCURRENCY: usize = 8;
+pub(crate) const SYNC_CONCURRENCY: usize = 8;
 
 /// A message to the sync engine's control thread.
 pub(crate) enum SyncMsg {