gitea-backup-pull: skip unmount (no error) if another process is busy on WSD2L840

rc.unassigned umount always does a lazy unmount and reports success
unconditionally, even with an open cwd on the mountpoint - it never
actually refuses on its own. Check with fuser -m first instead, and
just leave the disk mounted if anything is using it.

README.md

@@ -490,9 +490,23 @@ The SSH keypair needed no special handling: Unraid already auto-mirrors
   testing this: the script itself `cd`s into the mounted directory, so
   without explicitly `cd`ing back out first, the unmount would fail with
   "target is busy" (the shell's own working directory still being on the
-  device it's trying to unmount). Verified both directions: already
-  mounted → stays mounted after; not mounted → gets mounted, used, and
-  unmounted again, with no "busy" error.
+  device it's trying to unmount).
+- **Also checks for any *other* process still using the disk before
+  unmounting, and gives up quietly (no error) if it finds one.** This
+  turned out to be necessary rather than just nice-to-have: the
+  Unassigned Devices plugin's own `rc.unassigned umount` always does a
+  *lazy* (`-l`) unmount and reports "success" unconditionally — verified
+  live that it does **not** refuse, error, or even notice when another
+  process still has the mountpoint open (a single-device call is never
+  the "force" path that would otherwise kill blocking processes first).
+  So the script runs its own `fuser -m` check first and simply skips
+  calling `rc.unassigned umount` at all if anything turns up, logging a
+  plain informational line rather than an error and leaving the disk
+  mounted for next time. Verified all three states end-to-end: already
+  mounted before the run → untouched either way; not mounted, nothing
+  else using it → mounted, used, and cleanly unmounted again; not
+  mounted, but something else `cd`s into it mid-run → mounted, used, and
+  correctly left mounted with no error.
 
 **Restore path, if ever needed**: manual and deliberate — stop the target
 Gitea container, unpack the chosen dump's DB/repos/etc. into `/data` per