fix(security): address critical and high findings from security audit

Fix stored XSS in tasks (titles/subtasks) and settings (member list)
by applying escHtml(). Harden trust proxy to loopback default, add
OAuth state parameter for Google Calendar CSRF protection, sanitize
CSV export against formula injection, invalidate sessions on user
deletion, restrict usernames to alphanumeric chars, and require admin
role for calendar sync triggers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/index.js

@@ -60,8 +60,9 @@ app.use(helmet({
   } : false,
 }));
 
-// Trust Proxy für korrekte IP hinter Nginx
-app.set('trust proxy', 1);
+// Trust Proxy: nur aktivieren wenn ein Reverse Proxy vorgeschaltet ist (TRUST_PROXY env var).
+// Default 'loopback' akzeptiert nur X-Forwarded-For von localhost - verhindert IP-Spoofing.
+app.set('trust proxy', process.env.TRUST_PROXY || 'loopback');
 
 // --------------------------------------------------------
 // Request-Parsing