fix: document COLLAB_EMBEDDED_WS=true requirement for local self-hosting

[jessems]

Summary

• When running on localhost without COLLAB_EMBEDDED_WS=true, the server calculates the collab WebSocket URL as PORT+1 (4001 by default) rather than the actual server port (4000).
• Nothing runs on port 4001, so the editor silently stays in "connecting" state indefinitely with no error message surfaced to the user.
• The embedded collab runtime logs wsUrlBase=ws://localhost:4000/ws at startup, but the URL served to clients points elsewhere — the mismatch is invisible without inspecting the /collab-session API response.

Root cause

In server/routes.ts, resolveRequestScopedCollabWsBase defaults to appPort + 1 for loopback hosts unless COLLAB_EMBEDDED_WS is set, because in the hosted deployment the collab runtime runs as a separate process on a different port. For self-hosted local use, the runtime is always embedded and the env var must be set explicitly.

Fix

• Add COLLAB_EMBEDDED_WS=true to .env.example with an explanation.
• Update the README quickstart npm run serve command to include it.

Reproduction

npm install && npm run build
npm run serve   # without COLLAB_EMBEDDED_WS
# create a doc, open it in browser → "connecting" forever
curl http://localhost:4000/documents/<slug>/collab-session | grep collabWsUrl
# → ws://localhost:4001/ws  (wrong port, nothing listening)

Test plan

• COLLAB_EMBEDDED_WS=true npm run serve
• Confirm /collab-session returns ws://localhost:4000/ws
• Open editor — connects and loads document