quak helper list-missing-thumbnails: scans all files, fetches each
thumbnail from CDN, reports missing/empty ones.
quak helper fix-missing-thumbnails: downloads originals, generates
720px JPEG thumbnails via sharp, encrypts with secretstream, uploads
via presigned URL, registers via PUT /files/thumbnail. Supports
--file <ids> to fix specific files or auto-detects all missing.
Both commands support --json output. Live-tested against dev account.
