perf(images): refresh the listing when the monitor sees a new output (#348, PR 2)

The image monitor now reports every settled output (linked or not, also
when linking raised) with its resolved path through set_output_listener.
The API's notify_output_settled hops from the monitor's worker thread onto
the server's loop (call_soon_threadsafe) and does nothing before the loop is
known or after cleanup. mark_root_dirty matches the path against each
walk's resolved_root (normcase(realpath), computed in the executor by
walk_root, now its fourth value) and marks the root dirty.

A dirty root gets one background walk, no sooner than
rewalk_interval = min(15, max(5, 5 x last walk's duration)) after the
previous walk started (last_started_at / last_duration are recorded on
every walk). _schedule_walk is shared with PR 1's delayed stale refresh:
one deferred walk per root, never armed while a walk runs (the walk's end
re-checks dirty: the trailing walk), at least 0.5 s away, the earlier
deadline wins, and every walk start cancels it. A failed walk clears dirty.
Rescan semantics are unchanged.

__init__.py

@@ -149,6 +149,9 @@ try:
         # Every settled output's preview is queued for the API's background
         # worker (#249): the workspace never asks to generate thumbnails.
         _global_image_monitor.set_thumbnail_queue(_review_api.queue_capture_thumbnail)
+        # Every settled output walks its Images listing again (#348): the
+        # API hops from the monitor's thread onto the server's loop.
+        _global_image_monitor.set_output_listener(_review_api.notify_output_settled)
         _review_host = getattr(config.server_instance, "app", None)
         if _review_host is not None and hasattr(_review_host, "on_cleanup"):
 
@@ -157,6 +160,7 @@ try:
             ):
                 monitor.set_review_recorder(None)
                 monitor.set_thumbnail_queue(None)
+                monitor.set_output_listener(None)
                 api.close_auto_thumbnails(final=True)
 
             _review_host.on_cleanup.insert(0, _stop_review_recording)