We now have multiple versions of the swh-graph implementation around, as well as multiple versions of the compressed dataset. We should make those info human- and machine-accessible.

Practical suggestions:

• make the HTML API front-end say "this instance is running swh-graph X.Y.Z; the dataset version/timestamp is XXX"
• make the /graph/stats endpoint return the same info (speaking of which, we might take the change to rename it into something more general, e.g., /graph/about, with stats as dictionary key and other stuff as sibling keys

This will require making the dataset version/timestamp accessible in some machine-readable form on the on-disk graph representation. It might already exists, or not—I've no idea.