Add KEEP_LAST retention policy for backups

Add KEEP_LAST environment variable to always preserve the N most recent
backups regardless of age, preventing data loss when backups haven't
been created for extended periods. Defaults to 10 copies.

.env.example

@@ -1,6 +1,7 @@
 DB_FILE=/data/db.sqlite
 CRONTAB="00 06 * * *"
 DELETE_AFTER=10
+KEEP_LAST=10
 TZ=Asia/Novosibirsk
 MINIO_PATH=myminio://sqlite/master/
 MINIO_ACCOUNT_ID=account