Start Slack Install

Where to send an admin's browser to start a Slack install.

A one-field envelope rather than a bare string because the route must stay a JSON endpoint: GET /recipients/slack/install returns the URL, it does not 302 to it. A server-side redirect would force the front end into a full-page navigation, and this flow is opened in a popup in one place and a same-tab navigation in another — that choice belongs to the caller.

The URL carries the signed state minted by begin_install, which is the OAuth callback's only proof of which company is being installed for. It is therefore single-purpose and short-lived (a 10-minute TTL): do not cache this response, and do not reuse one URL for two install attempts.