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

src/metadata-backup.ts

@@ -140,8 +140,8 @@ const extractExif = async (
 // Dump every decrypted metadata layer the account holds into a directory tree
 // of plain JSON: account, per-collection, and per-file records including the
 // private and public magic metadata and (by default) the ML data. Collections
-// and files are enumerated from the library's cache rather than a fresh server
-// scan. Returns how many ML data requests failed; their files are still
+// and files are enumerated from the library's cache, which the caller refreshes
+// first. Returns how many ML data requests failed; their files are still
 // written, with `mlDataError` in place of `mlData`.
 export const runMetadataBackup = async (
     lib: Library,