fix(exit): stop running jobs before Python joins the executor threads (#376)

ComfyUI returns from main.py after Ctrl+C without closing its event loop,
and Python joins every executor thread before any atexit callback runs, so
a scan batch, thumbnail batch or model download in flight kept ComfyUI
alive until it ended (on Windows each ffmpeg up to its 30 s timeout).

The package now registers a threading._register_atexit hook (after
concurrent.futures', so it runs before that join) that sets an exit signal
and kills the running encoder processes. Job.cancelled follows the signal,
so every check a job already makes between files stops it at exit;
map_parallel starts no further item, the scan walk stops between folders and
a tagging model download stops before its next chunk. Without the internal
hook exit behaves as before.

py/api/admin.py

@@ -34,6 +34,11 @@ try:
 except ImportError:
     from utils.parallel import map_parallel
 
+try:
+    from ...utils import exit_signal
+except ImportError:
+    from utils import exit_signal
+
 try:
     from ...database.pair_ingestion import blank_positive, ingest_resolved_pair
     from ...database.run_attribution import (
@@ -118,6 +123,7 @@ def _collect_media_files(output_dirs, extensions):
         if not os.path.isdir(root):
             continue
         for current, dirnames, filenames in os.walk(root, followlinks=False):
+            exit_signal.checkpoint()  # #376: stop between directories at exit
             depth = 0 if current == root else len(Path(current).relative_to(root).parts)
             dirnames[:] = sorted(
                 d