fix: apply demo config to running LibreChat

[dustinhealy]

Summary

• reconcile prepare-demo.sh changes with an already-running LibreChat container instead of leaving stale environment variables active
• recreate only LibreChat when its running Compose hash differs, then verify the effective Langfuse URL and project keys without printing secrets
• add smoke coverage that deliberately transitions a running container from Cloud fixture settings back to the local Langfuse project
• document the standalone reconciliation helper for manual .env edits

Why

Docker containers do not receive .env changes after creation. A demo could therefore report that local Langfuse was configured while the running LibreChat process continued exporting traces to a previous Cloud target.

Validation

• bash -n scripts/reconcile-demo-runtime.sh scripts/prepare-demo.sh
• workflow YAML parsed successfully
• helper no-ops cleanly when the project is not running
• PR smoke test exercises the live Cloud-to-local transition