Map every connection to the workflows that reference it

Returns the reverse index connectionId -> [{id, name}, ...] for the caller's whole workspace — each credential's blast radius. Call it before deleting or replacing a connection to see what breaks.

Computed by scanning every workflow definition in the workspace for "connectionId": "<uuid>", so it covers triggers, steps, branch arms, loop bodies and error arms alike. A workflow referencing the same connection from several steps is listed ONCE.

IMPORTANT: a connection with no usage is simply ABSENT from the map — it is not present with an empty array. Treat a missing key as zero usage. A workspace where nothing references anything returns {"data": {}}.

The scan reads each workflow's DRAFT definition, so a reference that survives only in a published version and has been edited out of the draft does not appear here.

This route is matched before /api/connections/{id}, so usage is not a connection id.