Check if Slack OAuth is configured on this environment.
Returns {"available": true} when SLACK_CLIENT_ID is set, else
{"available": false}. The frontend uses this to conditionally render
the “Sign in with Slack” button. It does NOT build the authorize URL
itself — it calls POST /api/auth/slack/authorize which sets a
short-lived HttpOnly cookie carrying returnTo and returns the full
Slack URL for the browser to navigate to.