Mint a Sign-in-with-Slack invite URL initiated from settings, without Slack-DM context. The returned URL takes the user through PostHog login (already satisfied here), then to Slack OAuth, then back to our callback which writes the ``UserIntegration`` row. Without body params, falls back to the user's ``current_team`` and that team's first Slack ``Integration`` — works when there's exactly one linkable workspace. With ``team_id`` + ``slack_team_id``, links against the exact pair (what the frontend uses when a picker is shown). Refuses if the target team has no matching Slack workspace, if the feature flag is off for the workspace, or if the user is already linked to it.