Add selfupdate: replace the running program with the newest Gitea release

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

selfupdate/restart_unix.go

@@ -0,0 +1,13 @@
+//go:build !windows
+
+package selfupdate
+
+import (
+	"os"
+	"syscall"
+)
+
+// restart replaces this process with the new binary, with the same arguments and environment.
+func restart(exe string) error {
+	return syscall.Exec(exe, os.Args, os.Environ())
+}