release: v2.1.1

Version bumped in server.py, pyproject.toml, the PWA header and the
docs/api.md example; SW cache v124 -> v125.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'otaman-v124';
+const CACHE = 'otaman-v125';
 const URLS = [
   'index.html',
   'help.html',