feat(platform): build collaborative workspace tool

Start by evaluating HedgeDoc as the proposed foundation, then compare that approach with the current React SPA and the listed real-time, authentication, storage, and integration requirements. The issue provides no files or tests to run; a useful outcome would be a decided MVP scope and implementation direction after the membership system is stable.

Summary

Build a custom collaboration tool at `collab.us-rse.org` inspired by HackMD (real-time collaborative Markdown) and Notion (structured pages, databases, wikis). This replaces Slack Canvases and gives working groups a persistent space for living documents, meeting notes, project plans, and shared references.

Requirements

Core Features (MVP)

• Real-time collaborative Markdown editing (multiple cursors, conflict resolution)
• Document organization: workspaces per working group / affinity group
• Folder/page hierarchy within each workspace
• Version history (view and restore previous versions)
• Permissions: workspace-level (who can view/edit), document-level overrides
• Rich Markdown rendering (tables, code blocks, math/LaTeX, Mermaid diagrams)
• Export to PDF/Markdown/HTML

Workspace Structure

• One workspace per Working Group
• One workspace per Affinity Group
• Board workspace (private)
• Public workspace (community-wide shared docs)
• Personal workspace per member (private notes)

Authentication

• WorkOS SSO integration — same identity as us-rse.org and chat.us-rse.org
• Role mapping: WG chairs get workspace admin, members get editor access
• Guest/read-only access for public documents

Integration

• Link sharing (paste a collab.us-rse.org link in Zulip, get a rich preview)
• Embed documents in the main US-RSE site (e.g., meeting notes on WG pages)
• n8n integration (trigger workflows when documents are updated)
• Notification when a document you're subscribed to changes

Future Features (Post-MVP)

• Notion-style databases (tables with views, filters, sorts)
• Templates (meeting notes template, project proposal template)
• Comments/inline discussion on documents
• Offline editing with sync on reconnect

Context

This is a future build — not blocking the Slack migration. Priority order:

1. #1924 Zulip (chat) — immediate
2. #1925 n8n (workflows) — alongside Zulip
3. This issue (collaboration) — after membership system is stable

The current site is a React SPA. This tool could be a separate application or an integrated section of the main site, depending on complexity. HackMD's open-source edition (CodiMD/HedgeDoc) could serve as a foundation rather than building from scratch.

Implementation Notes

• HedgeDoc (formerly CodiMD) is the open-source HackMD. It provides real-time collaborative Markdown editing out of the box and is self-hostable. Evaluate whether to use it as-is, fork and customize, or build from scratch.
• Real-time collaboration requires operational transforms (OT) or CRDTs — libraries like Yjs or Automerge handle this.
• Consider Tiptap (ProseMirror-based) for a richer editor beyond pure Markdown.
• Postgres (NeonDB) for document storage, with JSONB for structured content.
• WebSocket server for real-time sync.