Relay Route

Record a thread→owner route created by a relay (local) mention.

When a mention runs on the desktop, cloud never sees the workspace, so the desktop reports the thread mapping here; without it, later replies in the thread have no route and cloud would drop them. First writer wins.

The caller may only record routes for a workspace it has a live Slack install in. The route is keyed on Slack coordinates alone, so without this check any authenticated API key could squat or mis-route another tenant's threads.