fix(fuse,core,gui): routine mirror scans no longer show as syncing

Every mirror pass, the two-minute poll included, reported a "Checking
{folder}" job, and the tray took any job for syncing, so its icon spun
every two minutes over folders with nothing to do.

JobItem gains a background flag (absent from older daemons, so false)
for housekeeping that changes nothing in the user's files: a pass still
scanning, the home index rebuild and the cache integrity check. The tray
keeps them in its menu line but only turns busy for transfers or a job
that is not background; a pass that queues work becomes a Syncing job
and still spins the icon.

The watcher also reconciles only the folder an event lies in, instead of
walking every mirror folder over one edit.

crates/pdfs-core/src/control.rs

@@ -1225,6 +1225,12 @@ pub struct JobItem {
     /// Steps known to need doing, or `0` when unknown (indeterminate progress).
     /// May *grow* mid-job as more work is discovered.
     pub total: u64,
+    /// Housekeeping that changes nothing in the user's files: a mirror folder
+    /// scan that finds nothing to do yet, an index rebuild, a cache check. Front
+    /// ends may show it, but should not report the drive as busy over it. Absent
+    /// from older daemons, which never set it.
+    #[serde(default)]
+    pub background: bool,
 }
 
 /// One in-flight transfer in a [`Response::Transfers`] snapshot.