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

README.md

@@ -480,7 +480,13 @@ on. The exit code is non-zero if any ML data request failed.
 only, because the bundled decoder (`jpeg-js`) decodes only JPEG. A non-JPEG
 image (PNG, HEIC) or a video is reported as `skipped` (unsupported format), kept
 distinct from a `failed` repair, and does not affect the exit code; a genuine
-failure still exits non-zero.
+failure still exits non-zero. 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
+for the file. So a file another account owns, in an album shared with you, is
+skipped by both thumbnail helpers without being fetched, and the fixer skips a
+file whose recorded thumbnail size is 0 or unknown. Otherwise the fixer lowers
+the quality and size of the thumbnail until it fits, and skips the file if even
+the smallest does not.
 
 ### Backup layout