Limit request sizes, answer 413, stop cleanly on SIGTERM

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaC9HmtHMdRP8dAbN4wAKj

admin.js

@@ -4,7 +4,7 @@ const users = require('./users');
 const auth = require('./auth');
 const audit = require('./audit');
 
-const readBody = req => new Promise((ok, no) => { const c = []; req.on('data', d => c.push(d)); req.on('end', () => ok(Buffer.concat(c).toString())); req.on('error', no); });
+const { readBody } = require('./body');
 const json = (res, code, obj) => { res.writeHead(code, { 'content-type': 'application/json' }); res.end(JSON.stringify(obj)); };
 const row = u => ({ ...users.publicView(u), tabsChosen: u.tabs || [], passkeys: auth.passkeyCount(u.id) });