Initial scaffold: Borg engine + docs (Layer 1 milestone)

apiscp-borg is a sibling of apiscp-kopia built on BorgBackup. This first
milestone lands the project scaffold and the Layer 1 engine:
- borg-apiscp-backup: per-site named archives (siteN-shadow/info/db), fresh
  per-site DB dumps via ApisCP site-context export, system + custom archives,
  borg prune retention per prefix, Prometheus metrics, email notifications.
- apiscp-borg-common.sh: logging, mail, site/owner helpers.
- config example, systemd service+timer, install.sh, README, DESIGN.

Borg preserves POSIX ACLs and xattrs natively, so (unlike the kopia engine) no
metadata sidecar is required; DESIGN.md records how Borg reshapes the design.
Repository/restore tools, Layer 2 panel integration, hooks, uninstall, and the
full reference are the next milestones.

systemd/borg-apiscp-backup.timer

@@ -0,0 +1,11 @@
+[Unit]
+Description=apiscp-borg nightly backup timer
+Documentation=https://git.discworld.casa/laurence/apiscp-borg
+
+[Timer]
+OnCalendar=*-*-* 03:30:00
+RandomizedDelaySec=1800
+Persistent=true
+
+[Install]
+WantedBy=timers.target