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.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=apiscp-borg backup run (ACL/xattr-aware BorgBackup engine for ApisCP)
+Documentation=https://git.discworld.casa/laurence/apiscp-borg
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+Nice=10
+IOSchedulingClass=idle
+ExecStart=/usr/local/bin/borg-apiscp-backup
+# The engine holds its own lock and writes metrics on exit.
+TimeoutStartSec=0