Refresh before backup-metadata and the thumbnail helpers answer (closes #100)

backup-metadata, helper list-missing-thumbnails and helper
fix-missing-thumbnails now await lib.fresh() before reading the library,
as collections, files, get and get-thumb already do. A file added since
the cache was written is included, and a failed refresh is thrown, so
the CLI prints one line and exits 1 instead of answering from a stale
or empty cache. The README lists them among the commands that refresh
first.

Model: opus-5-5

README.md

@@ -449,10 +449,13 @@ quak helper fix-missing-thumbnails [--file ids] generate + upload missing thumbn
 ```
 
 Every command runs on the same cache-backed library. The read commands —
-`collections`, `files`, `get`, and `get-thumb` — force a fresh server round-trip
-before they answer, so they report current account state rather than whatever
-the cache last held. `--cache-dir` overrides where the cache lives; without it
-each account gets its own directory under the per-user cache path.
+`collections`, `files`, `get`, `get-thumb`, `backup-metadata`,
+`helper list-missing-thumbnails` and `helper fix-missing-thumbnails` — force a
+fresh server round-trip before they answer, so they report current account state
+rather than whatever the cache last held. If that round-trip fails, the command
+prints the error on one line and exits 1. `--cache-dir` overrides where the
+cache lives; without it each account gets its own directory under the per-user
+cache path.
 
 `get` and `get-thumb` resolve the file by ID directly, so `--collection` is
 accepted for backward compatibility but ignored. `backup-metadata --exif` (alias