When the dev file-watcher and an upload endpoint both trigger merge_all on the
same user directory at the same time, the shutil.rmtree/_merged/activities/
sequence collides: one thread deletes the directory while the other is mid-walk,
causing ENOTEMPTY or ENOENT crashes.
Added a per-user-dir threading.Lock (_merge_lock) at the top of merge.py so
all callers (upload endpoints, watcher, admin rebuild) are serialized without
changing any call site. Also made the rmtree ignore_errors=True and the
symlink_to conditional to survive any stale leftover from a prior crash.
Davide Scaini
cfb3ba5871