backups: seconds in the name, so two in one minute don't overwrite; update.sh prints the api's log on a failed check

docs/admin-guide/maintenance.md

@@ -100,7 +100,7 @@ A later rebuild without `--build-arg` goes back to the version the code names.
 
 ![Settings → Backups](img/settings-backups.png)
 
-A backup (`data/backups/selfphoneng-YYYYMMDD-HHMM.tar.gz`) holds:
+A backup (`data/backups/selfphoneng-YYYYMMDD-HHMMSS.tar.gz`) holds:
 
 - the database: users, extensions, numbers, settings, call history;
 - voicemail;

docs/spec/04-backend-api.md

@@ -477,7 +477,7 @@ whole minutes). Each job has a running flag; an error is logged, not fatal.
   backup emails them too (08 §3.2). Email not set up: logged, nothing else.
 
 ### 8.1 Backups
-Folder `/var/lib/selfphoneng/backups/selfphoneng-YYYYMMDD-HHMM.tar.gz` containing:
+Folder `/var/lib/selfphoneng/backups/selfphoneng-YYYYMMDD-HHMMSS.tar.gz` containing:
 `db.sql` (`pg_dump --no-owner`), `voicemail/`, `sounds-custom/`, `moh/`,
 `manifest.json`. Do **not** include `asterisk-generated/`: it holds plaintext SIP passwords and is rebuilt from the DB on start.
 `manifest.json` is `{version, createdAt, includesRecordings}`. The backup file mode is 0600. Recordings are optional