Skip thumbnail repairs the server always refuses (closes #109)

The server accepts a new thumbnail only from the file's owner and only when
it is no larger than the thumbnail size it records. Both thumbnail helpers
now skip files another account owns without fetching them. The fixer skips
a file whose recorded thumbnail size is 0 or unknown before downloading it,
and otherwise tries smaller encodings until the encrypted thumbnail fits,
skipping the file if none does.

Model: opus-5-5

TODO.md

@@ -18,6 +18,13 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Stopped `helper fix-missing-thumbnails` retrying files the server
+  always refuses (issue 109). Both thumbnail helpers skip a file another account
+  owns without fetching it. The fixer skips a file whose recorded thumbnail size
+  is 0 or unknown before downloading it, and otherwise tries smaller encodings
+  (720 px quality 50 down to 160 px quality 20) until the encrypted thumbnail is
+  no larger than that size, skipping the file if none fits.
+
 - 2026-09-23: Checked downloaded originals against their recorded content hash
   (issue 68). `downloadFile`, which `quak get`, the content cache and backup all
   use, hashes the decrypted bytes (unkeyed BLAKE2b-512, standard base64) and