feat: author pages with /authors/:id route (0.16.0)

- `GET /v1/authors/:id` returns `AuthorWithCount` (id, name, manga_count).
- `GET /v1/authors/:id/mangas` paged works by that author.
- `GET /v1/authors?search=` autocomplete (already used by Phase 1 forms;
  now formally exposed).
- New `/authors/:id` page on the frontend; author chips on the manga
  detail page (added in Phase 1) now link to a real page.
- Extracts `lib/components/MangaCard.svelte` — already used by the home
  page, ready for the collection page in Phase 3.

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

backend/src/api/mod.rs

@@ -1,4 +1,5 @@
 pub mod auth;
+pub mod authors;
 pub mod bookmarks;
 pub mod chapters;
 pub mod files;
@@ -22,4 +23,5 @@ pub fn routes() -> Router<AppState> {
         .merge(bookmarks::routes())
         .merge(genres::routes())
         .merge(tags::routes())
+        .merge(authors::routes())
 }