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/files.rs

@@ -493,9 +493,11 @@ pub async fn list_photos<TagD: TagDao, FS: FileSystemAccess>(
                 .set_attribute(KeyValue::new("total_count", total_count.to_string()));
             span_context.span().set_status(Status::Ok);
 
+            let photo_libraries = vec![scoped_library.id; tagged_files.len()];
             HttpResponse::Ok().json(PhotosResponse {
                 photos: tagged_files,
                 dirs: vec![],
+                photo_libraries,
                 total_count: pagination_metadata.0,
                 has_more: pagination_metadata.1,
                 next_offset: pagination_metadata.2,
@@ -778,9 +780,11 @@ pub async fn list_photos<TagD: TagDao, FS: FileSystemAccess>(
                 .set_attribute(KeyValue::new("total_count", total_count.to_string()));
             span_context.span().set_status(Status::Ok);
 
+            let photo_libraries = vec![scoped_library.id; response_files.len()];
             HttpResponse::Ok().json(PhotosResponse {
                 photos: response_files,
                 dirs,
+                photo_libraries,
                 total_count: pagination_metadata.0,
                 has_more: pagination_metadata.1,
                 next_offset: pagination_metadata.2,