Get the plan-gating payload for a workspace (any member)

Slim subscription + usage + feature map + trial info + resolved plan — everything the web app needs to pre-flight plan limits and feature gates, and to render the trial/past-due banners. subscription.plan is what the workspace is buying; resolvedPlan is what the server actually enforces against (they differ during a trial, which resolves to ENTERPRISE), so upgrade prompts should rank against resolvedPlan. Readable by any workspace member. For the full billing payload (billing address, tax id, Stripe details), see GET /billing.