Make backup wait for the server refresh and fail when it fails (closes #99)

lib.backup() refreshed through the background loop's refresh, which returns
at once when one is already running and swallows a failure, so a backup
could run on the previous file list, or on an empty cache, and exit 0. It
now uses the refresh fresh() uses: it joins a running refresh or starts
one, and rejects before touching any file when it fails. The CLI's error
wrapper prints that as one line and exits 1.

Model: opus-5-5

TODO.md

@@ -18,6 +18,13 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: `quak backup` waits for the server refresh and fails when it fails
+  (issue 99). `lib.backup()` joins a refresh already running or starts one, as
+  `fresh()` does, and rejects before touching any file when it fails, leaving
+  `failures.json` as it was, so `quak backup` prints the error as one line and
+  exits 1 instead of backing up the previous run's file list, or nothing, and
+  exiting 0.
+
 - 2026-09-23: CLI errors print a message instead of a stack trace (issue 102).
   An error a command throws is printed as one `quak: MESSAGE` line on stderr and
   the CLI exits 1 once output has drained. The wrapper that does this moved from