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>

go.mod

@@ -3,7 +3,7 @@ module git.veillax.com/Veillax/blacktip
 go 1.27.3
 
 require (
-	git.veillax.com/Veillax/reef v0.1.0
+	git.veillax.com/Veillax/reef v0.1.3
 	github.com/emersion/go-msgauth v0.7.0
 	golang.org/x/crypto v0.58.0
 )