Get Decision Lane Counts

Counts per review lane, from the acceptance join.

Not substitutable with /decisions/counts. That endpoint groups the status column, which is the extractor's projection kept in step for readers that predate the acceptance split, so its active and this endpoint's active answer different questions: a record can be stored active and have no acceptance at all. A tab row labelled from the other one's answer would overcount every lane it names.

Declared above /{decision_index} for the same reason /counts is: FastAPI matches in declaration order, and lane-counts would otherwise be parsed as an integer index and 422.

A missing decisions artifact 404s exactly as the list endpoint does.