A 64-bit difference hash of each thumbnail, stored in the new
photos.similar_hash column (schema 34), and the queries the similar-photo
finder needs: what is left to hash, progress, and the sets of shots that
look alike. Sets made only of byte-identical copies are left to the
duplicate finder. A refresh keeps a hash unless the photo's bytes changed.
The daemon pass, the control request and the GUI are not built yet; see
PLAN.md.
