Make thumbnail generation atomic and warm it on import

- write thumbnails to a .part file and os.replace() them, so concurrent
  requests never read a half-written JPEG
- a stale thumbnail plus a vanished source no longer raises through the
  request (getmtime on a missing file returned 500); it falls back cleanly
- ensure_thumbnail(item) warms the preview when a file is indexed, keeping
  image decoding out of the request path

backend/apps/library/uploads.py

@@ -128,6 +128,12 @@ def complete_temp_upload(temp, download_url=None):
         services.ensure_visual_hashes(item)
         temp.file.delete(save=False)
 
+    # Warm the preview while the import is still off the request path.
+    try:
+        services.ensure_thumbnail(item)
+    except Exception:  # noqa: BLE001 - a preview must not fail the import
+        logger.exception("Could not warm the thumbnail for J-%s", item.id)
+
     temp.library_item = item
     temp.status = TempUpload.STATUS_COMPLETED