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.

docs/CHANGELOG.md

@@ -11,6 +11,9 @@ scratch (user data in `staging/` and `recovery/` is never touched by this).
 
 ## [Unreleased]
 
+Schema: **34**. Photos gain a `similar_hash` column for the coming similar-photo finder. It
+stays empty until the daemon hashes thumbnails, so updating reads and downloads nothing.
+
 ## [2.3.0] — 2026-09-28
 
 Schema: **33**. Photos gain their location; the first timeline refresh after updating reads