feat(thumbnails): keep previews in one folder, set from config.json (#320)

thumbnails.location in config.json can now name one folder for every
source's previews: <folder>/<source key>/<rel>_thumb<ext>. A source key is
a readable slug plus a short hash of the source's identity, stored in the
folder's marker (.promptmanager-thumbnails.json) with the install that
made it. Serving, on-demand and capture writes, listings, clear, delete,
duplicates, the temp sweep and History covers all follow it; a leftover in
<source>/thumbnails is never served and other text under thumbnails/ is
refused.

A missing, unplugged or foreign folder is unavailable (a probe checks it
every 5 s within 2 s; a stalled or failing store call flips it at once):
originals are served, nothing is written anywhere, the folder is never
re-created, and generate, convert and clear answer 409
location-unavailable. A bad config value is unavailable too, never a
fall-back to the sources.

No UI yet: POST /settings refuses thumbnail_location (409
location-needs-move); GET /settings reports it with its path, the
thumbnail status without one. Sources inside or around the folder and a
library move into it are refused.

py/api/thumbnail_settings.py

@@ -221,7 +221,12 @@ def _owned(root, rel_path, preferred):
     if store is None:
         return iter(())
     stem = store / preview_stem(rel_path)
-    return owned_in_folder(stem.parent, stem.name, rel_path.suffix, preferred)
+    found = owned_in_folder(stem.parent, stem.name, rel_path.suffix, preferred)
+    if preview_store.current().mode == preview_store.SOURCE_MODE:
+        return found
+    # Folder mode: timed, so a stalled share marks the folder unreachable.
+    with preview_store.store_io():
+        return iter(list(found))
 
 
 def owned_thumbnails(root, rel_path, preferred=None):