feat: add album scope for similar image search

Similar search scoping:
- Add current_album as a similar-scope option and remember the source album when similar or region searches are launched from an album.
- Route gallery and lightbox similar actions through scope-aware store helpers so Album/Folder/All choices are applied consistently.
- Keep pagination and scope toggles working for both whole-image and region-search result sets.

Backend filtering:
- Extend find_similar_images and find_similar_by_region params with album_id, giving album scope precedence over folder scope.
- Add album_membership filtering for HNSW whole-image search and brute-force crop embedding search.

CHANGELOG.md

@@ -35,6 +35,9 @@ aims to follow [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
   whole library.
 - **Reorderable albums** — drag albums in the sidebar (hover the row for the
   drag handle) to set their order, which persists across sessions.
+- **Album-scoped similar search** — when finding visually similar images or
+  searching by a selected region from an album, you can now keep results scoped
+  to that album, switch back to the source folder, or search all media.
 - **What's New** — after updating, Phokus now greets you with a "What's new"
   toast that opens an in-app release-notes screen for the new version, with the
   changes grouped into collapsible Added / Changed / Fixed sections. It's