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.

src/components/Toolbar.tsx

@@ -162,6 +162,7 @@ export function Toolbar() {
   const setFailedTaggingOnly = useGalleryStore((state) => state.setFailedTaggingOnly);
   const similarScope = useGalleryStore((state) => state.similarScope);
   const setSimilarScope = useGalleryStore((state) => state.setSimilarScope);
+  const similarSourceAlbumId = useGalleryStore((state) => state.similarSourceAlbumId);
   const mediaJobProgress = useGalleryStore((state) => state.mediaJobProgress);
   const zoomPreset = useGalleryStore((state) => state.zoomPreset);
   const setZoomPreset = useGalleryStore((state) => state.setZoomPreset);
@@ -187,6 +188,12 @@ export function Toolbar() {
   const hasActiveSearch = search.trim().length > 0;
   const parsedSearch = parseSearchValue(composeSearchValue(searchCommand, searchQuery));
   const isSimilarResults = collectionTitle === "Similar Images";
+  // Album scope is offered while browsing an album (where it's the default) and
+  // while viewing similar/region results that were launched from an album.
+  const showAlbumScope =
+    activeView === "album" ||
+    (similarSourceAlbumId !== null &&
+      (collectionTitle === "Similar Images" || collectionTitle === "Region Search Results"));
 
   // If current sort is video-only but we switched away from video filter, reset to date_desc
   useEffect(() => {
@@ -450,6 +457,9 @@ export function Toolbar() {
         <FilterPill label="Favorites" active={favoritesOnly} onClick={() => { setFavoritesOnly(!favoritesOnly); setFailedEmbeddingsOnly(false); setFailedTaggingOnly(false); }} />
         <FilterPill label="Rated" active={minimumRating === 1} onClick={() => { setMinimumRating(minimumRating === 1 ? 0 : 1); setFailedEmbeddingsOnly(false); setFailedTaggingOnly(false); }} />
         <FilterPill label="4★+" active={minimumRating === 4} onClick={() => { setMinimumRating(minimumRating === 4 ? 0 : 4); setFailedEmbeddingsOnly(false); setFailedTaggingOnly(false); }} />
+        {showAlbumScope ? (
+          <FilterPill label="Similar: Album" active={similarScope === "current_album"} onClick={() => setSimilarScope("current_album")} />
+        ) : null}
         <FilterPill label="Similar: Folder" active={similarScope === "current_folder"} onClick={() => setSimilarScope("current_folder")} />
         <FilterPill label="Similar: All" active={similarScope === "all_media"} onClick={() => setSimilarScope("all_media")} />
         {hasAnyFailedEmbeddings ? (
@@ -468,7 +478,7 @@ export function Toolbar() {
             onClick={() => setFailedTaggingOnly(!failedTaggingOnly)}
           />
         ) : null}
-        {isSimilarResults ? <span className="ml-2 text-[11px] text-gray-500">Current similar scope: {similarScope === "current_folder" ? "current folder" : "all media"}</span> : null}
+        {isSimilarResults ? <span className="ml-2 text-[11px] text-gray-500">Current similar scope: {similarScope === "current_album" ? "this album" : similarScope === "current_folder" ? "current folder" : "all media"}</span> : null}
       </div>
     </div>
   );