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>

.env.example

@@ -18,3 +18,7 @@ BLACKTIP_TOKEN=
 #BLACKTIP_TRAEFIK=/usr/local/lib/blacktip/traefik
 #BLACKTIP_ACME_DNS=cloudflare
 #CF_DNS_API_TOKEN=
+
+# Check for new releases every few hours and update and restart by itself (Linux and macOS).
+# setup.sh turns this on; remove it to update by hand.
+#BLACKTIP_AUTO_UPDATE=1