docs: add plan for collaborative note editing

Server-authoritative operation log with per-block versions and 20s
block leases, conflict copies instead of data loss, IndexedDB queue
for offline use, HTTP-based sync with WebSocket/SSE/long-poll wakeups,
unified note/folder permissions, and per-user undo.

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

Doku/README.md

@@ -18,6 +18,7 @@ Referenz für Entwickler und Administratoren. Alle Angaben wurden gegen den Code
 | [reverse-proxy.md](reverse-proxy.md) | Nginx, Traefik, Caddy, Cloudflare |
 | [theming.md](theming.md) | Themes und Design-Variablen |
 | [flaws.md](flaws.md) | Bekannte Schwachstellen und technische Schulden |
+| [plans/kollaboration-notizen.md](plans/kollaboration-notizen.md) | Plan: gemeinsames Bearbeiten von Notizen |
 
 ## Überblick in 60 Sekunden