feat(core): perceptual hashes for finding similar photos

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.

crates/pdfs-core/src/lib.rs

@@ -20,6 +20,7 @@ pub mod sdkcache;
 pub mod search;
 pub mod service;
 pub mod shell;
+pub mod similar;
 pub mod syncignore;
 pub mod takeout;
 pub mod tray;