Failed runs end with one fatal line; failure lines are errors

A command that exits 1 because files or albums failed now ends with one
fatal line saying how many failed and where to look, so -q never leaves
a non-zero exit unexplained: backup points to failures.json, backup-metadata
to mlDataError in each file's JSON, and fix-missing-thumbnails to its
Failed files list. The progress callbacks of the backup, the metadata
backup and the thumbnail helpers take a level, so every line that reports
a failure is logged at error. The library's operation names are shown in
plain words. The README says which lines are error and warning, and which
summary lines --json replaces.

Model: opus-5-5

TODO.md

@@ -36,7 +36,10 @@ declares one.
   end of each refresh and ML data fetch; trace adds each request, which
   `ApiClient` reports through the new `onRequest`. `--json` no longer keeps the
   progress lines off stderr, so `backup` and `--json` runs log a refresh's
-  progress too.
+  progress too. Every line that reports a failed file or album is at error,
+  which the progress callbacks of `backup`, `backup-metadata` and the thumbnail
+  helpers now say, and a command that exits 1 because files or albums failed
+  ends with one fatal line saying how many and where to look.
 
 - 2026-10-18: `quak files` takes the collection ID as an optional argument
   instead of the `--collection` option, which is gone