Add opt-in auto-update from Gitea releases

The program now lives in /usr/local/lib/blacktip and is owned by its service user so it can replace itself.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -54,6 +54,10 @@ sudo systemctl start blacktip
 
 Use `linux_arm64` or `linux_arm` for Raspberry Pi and other ARM machines. To upgrade, repeat the same steps: your settings and data are kept. `sudo ./install.sh --uninstall` removes the service and binary but leaves `/etc/blacktip` and `/var/lib/blacktip` alone. `blacktip -version` tells you which build is running.
 
+**Updates**
+
+Set `BLACKTIP_AUTO_UPDATE=1` (or pass `-auto-update`) and Blacktip checks this server for a newer release every few hours, verifies it against `SHA256SUMS`, checks that the new binary runs, swaps it in and restarts itself. It only follows full releases (never pre-releases), never downgrades, and keeps the old binary if anything looks wrong. `setup.sh` turns it on. It is not available on Windows, and it is for the systemd and plain-binary installs: with Docker, pull a new image instead.
+
 **With Docker**
 
 ```