Slack-formatted webhooks; workspace invite management

Added

• Slack delivery format for webhooks — set format: "slack" on a subscription and point it at an incoming-webhook URL to receive releases as Slack Block Kit cards; the default HMAC-signed JSON delivery path is unchanged.
• Workspaces now support member and invitation management — invite teammates by email, toggle member/admin roles, remove or leave a workspace, and accept or decline invitations from the emailed link.

Changed

• Digest email subjects now include the covered date — daily digests show "Jun 24, 2026" and weekly digests show "week of Jun 17, 2026" alongside the update count, making threads easier to find in your inbox.