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.

README.md

@@ -0,0 +1,53 @@
+# apiscp-borg
+
+A [BorgBackup](https://www.borgbackup.org/) backup engine for
+[ApisCP](https://apiscp.com), plus native panel/CLI integration, packaged so it
+survives ApisCP upgrades. Sibling project to
+[apiscp-kopia](https://git.discworld.casa/laurence/apiscp-kopia): same
+architecture, different backend.
+
+## Why Borg, and why a plugin
+
+Borg is a fast, deduplicating, compressing, encrypted backup tool. Unlike kopia,
+**Borg preserves POSIX ACLs and extended attributes natively**, which is exactly
+the metadata ApisCP encodes account permissions in. So a plugin is not needed to
+work around lost metadata (the reason apiscp-kopia exists). The value here is:
+
+- runs ApisCP's `backup_dbs.php` before archiving, so per-site database dumps
+  are current instead of a day stale;
+- archives each site as its own named set (`siteN-shadow-...`, `siteN-db-...`),
+  enabling single-site restore and per-site retention;
+- captures per-site databases through ApisCP's site-context export (a host-level
+  dump cannot see them);
+- wires retention (`borg prune`), maintenance (`borg compact`), integrity
+  (`borg check`), scheduling, email notifications, one-time key backup, and both
+  an appliance-admin and a site-owner GUI into ApisCP.
+
+See `docs/DESIGN.md` for how Borg changes the design relative to apiscp-kopia.
+
+## Architecture
+
+Two decoupled layers, both upgrade-safe:
+
+- **Layer 1: the engine.** A standalone POSIX-sh program on a systemd timer,
+  with no coupling to ApisCP's PHP internals. `bin/borg-apiscp-backup` plus
+  `borg-apiscp-repo` and `borg-apiscp-restore`.
+- **Layer 2: native integration.** An ApisCP module, GUI apps, and account hooks
+  under `/usr/local/apnscp/config/custom`, which survive `upcp`.
+
+## Requirements
+
+- ApisCP (any recent release)
+- `borgbackup` (`borg`) 1.2+ on the host (and on the remote for ssh backends)
+- a filesystem that carries ACLs and xattrs (default on ext4/xfs/btrfs)
+
+## Status
+
+Early scaffold. The Layer 1 engine is the first milestone; the repository and
+restore tools, Layer 2 panel integration (module + two GUIs + hooks), install /
+uninstall, and full reference documentation are being built out to track the
+apiscp-kopia feature set. Not yet ready for production use.
+
+## License
+
+MIT. See `LICENSE`.