feat(backup): automated daily backup script + systemd timer

- scripts/backup.sh: pg_dump + project tar, S3 upload (optional),
  local retention (7 days), dry-run support
- systemd/nexus-backup.service + nexus-backup.timer: daily at 02:00 UTC
- AWS CLI v2 installed locally in tools/ for S3 uploads

systemd/nexus-backup.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Daily Nexus backup at 02:00 UTC
+
+[Timer]
+OnCalendar=*-*-* 02:00:00
+Persistent=true
+
+[Install]
+WantedBy=timers.target