perf(images): start a stale listing's refresh walk a second after the read (#348)

After the faster walk, a read that found the listing stale still took
0.5-1.0 s against 0.07 s: it was served from the old listing but ran beside
the refresh walk it had just started, sharing the GIL. The refresh now starts
REFRESH_DELAY (1 s) after that read through a per-root rewalk_handle, so the
page's own requests finish first. One pending refresh per root and none while
a walk runs; any walk start (a cold read, Rescan, the warm-up) cancels a
pending one; on_cleanup cancels them before its first await.

py/api/__init__.py

@@ -548,6 +548,11 @@ class PromptManagerAPI(
     async def cleanup_gallery_listings(self, app=None):
         """Stop the background listing walks (aiohttp on_cleanup)."""
         self._workspace_warm_enabled = False
+        # Before any await, so no delayed refresh can start a walk meanwhile.
+        for record in self._root_listings.values():
+            handle, record.rewalk_handle = record.rewalk_handle, None
+            if handle is not None:
+                handle.cancel()
         warm, self._workspace_warm_task = self._workspace_warm_task, None
         if warm is not None:
             warm.cancel()