Verify auth and get the targeted space, user, membership, and all spaces

The space this request targets, the acting user, their membership, and every space the user belongs to (spaces). Open source returns the local space/user; the cloud resolves the real ones from the user API key — when spaces has more than one entry, target a specific space with the x-busabase-space header instead of relying on the default. Next: call playbooks search with the user's intent before other work.