Delete a workspace (two-step; ?confirm=true)

Permanently deletes a workspace and its entire entity tree (tables, rows, cells, sequences, chat history, documents). Follows the v2 deletes convention — deliberately two-step so it can't fire on a single ambiguous instruction:

1. Call without ?confirm=true → no deletion. Returns HTTP 200 with the impact preview { workspaceId, name, confirmationRequired: true, willDelete: { tables, rows } }. The caller is expected to check with the user before proceeding.
2. After the user agrees, retry with ?confirm=true → performs the cascade soft-delete and returns { workspaceId, name, deleted: true }.

A missing / cross-org / already-deleted workspace returns 404 WORKSPACE_NOT_FOUND.