Slack workspace mismatch detection and reconnection

Goal: a member whose Slack link no longer matches the org's workspace is told and can fix it; Slack delivery checks the right workspace. - Profile › Alerts shows "Reconnect Slack" on a workspace mismatch. - Add to Slack's `off_plan` error is shown somewhere visible. - `slack_link.issue_code` raises a domain error mapped in `routes/auth.py`. - Short per-team cache for `slack_workspaces.for_team` in `slack_socket`. - Notify outbox carries the team checked at enqueue and refuses on mismatch. Out of scope: multiple workspaces per org; card designs. Done means: a test per item; quick gate green.

Slack workspace mismatch: Profile › Alerts "Reconnect Slack", off_plan banner on org page, SlackNotConnected→409, per-team for_team cache, outbox team check. Quick gate + tests green; visual check done; PR #6628 queued in lane.