Request and manage on-demand preview environments from Slack

Preview environments can now be requested directly from Slack. Operators can use the /preview command to create and delete isolated environments tied to a specific PR or commit, with TTL and idle-time cleanup. Each preview includes its own in-cluster analytics and object storage, and can optionally share a Kura cache mesh across all accounts in that preview. The original Slack card is updated when the preview finishes or fails, so the requester sees the live preview link or the failure state without leaving Slack.