feat: include per-photo library id in /photos response

Adds a parallel `photo_libraries: Vec<i32>` array alongside `photos`
in `PhotosResponse` so clients can render per-thumbnail badges.
Populated with the scoped library id at the two main return sites;
left empty for `/favorites` since favorites are library-agnostic.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -970,9 +970,12 @@ async fn favorites(
                 .collect::<Vec<String>>();
 
             span.set_status(Status::Ok);
+            // Favorites are library-agnostic (shared by rel_path), so we
+            // intentionally leave photo_libraries empty to signal "no badge".
             HttpResponse::Ok().json(PhotosResponse {
                 photos: favorites,
                 dirs: Vec::new(),
+                photo_libraries: Vec::new(),
                 total_count: None,
                 has_more: None,
                 next_offset: None,