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.

__init__.py

@@ -355,6 +355,31 @@ async def _stop_background_work_on_cleanup(app):
     _stop_background_work()
 
 
+def _kill_encoders_at_exit():
+    """Run by the exit hook before Python joins the executor threads (#376):
+    a JPEG XL or ffmpeg encoder (no console on Windows, so Ctrl+C never
+    reaches it) is killed instead of being waited for."""
+    from .py.api.thumbnail_encode import shutdown_encoders
+
+    shutdown_encoders()
+
+
+try:
+    # Before the executor join: running jobs stop at their next file, item or
+    # chunk (#376). The teardown below still runs from atexit afterwards.
+    from .utils import exit_signal as _exit_signal
+
+    _exit_signal.install(_kill_encoders_at_exit)
+except Exception as e:
+    try:
+        from .utils.logging_config import get_logger
+
+        get_logger("prompt_manager.init").error(
+            f"Failed to register the exit signal: {e}"
+        )
+    except Exception:
+        pass
+
 try:
     atexit.register(_stop_background_work)
     from .py.config import server_instance as _teardown_server