README: guard the daily cron scan against an unmounted disk (closes #175)

A disk that is not mounted leaves its mount point an empty directory.
scan then deletes every record under it, and the next scan reads every
file again. The README now says so, and its cron line runs scan only
when mountpoint reports /srv as mounted.

Model: opus-5-5

TODO.md

@@ -29,6 +29,9 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- `README.md` says what a disk that is not mounted does to the database. The
+  daily `cron` line runs `scan` only when `mountpoint` reports `/srv` as mounted
+  (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/175)
 - The usage line of `scan` no longer ends in `[flags]`. It is the line
   `README.md` shows under "Subcommands" (2026-11-11,
   https://git.eeqj.de/sneak/sfdupes/issues/169)