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_auto.py

@@ -175,7 +175,7 @@ class DeferredPreview:
 
     def resolve(self):
         """The preview's path now, or None when nothing may be written."""
-        dst = media_preview_path(self.root, self.rel)
+        dst = media_preview_path(self.root, self.rel, create=True)
         if dst is None:
             return None
         if self.strict:
@@ -298,8 +298,12 @@ def _thumbnail_source(root, parts):
     if in_trash_below_roots(found, [root]):
         return None
     rel = Path(*folder_parts, found.name)
-    dst = media_preview_path(root, rel)
-    if dst is None or dst != _current_preview(root, parts):
+    dst = media_preview_path(root, rel, create=True)
+    if dst is None:
+        # Nowhere to write (the thumbnail folder is unavailable): the
+        # original is served and nothing is written.
+        return found, None, found.suffix.lower() in VIDEO_EXTENSIONS
+    if dst != _current_preview(root, parts):
         return None
     if not preview_store.owns(root, dst):
         dst = None
@@ -745,6 +749,8 @@ class AutoThumbnailMixin:
             pool.shutdown(wait=False, cancel_futures=True)
         if final:
             shutdown_encoders()  # kill JPEG XL / ffmpeg encoders still running
+            # The thumbnail folder's probe (#320), bounded like the rest.
+            closed = preview_store.stop_probe() and closed
         return closed
 
     def _auto_thumbnail_failures(self):