perf: precompute mangas.sort_author for the author sort

?sort=author used a correlated min(lower(a.name)) subquery as the ORDER BY
key, evaluated per filter-matching row before LIMIT. Materialize it into
mangas.sort_author (indexed by mangas_sort_author_idx), maintained by triggers
on manga_authors and backfilled in the migration, so the sort is a plain
indexed column read. Author names are immutable, so only join changes matter.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/repo/manga.rs

@@ -168,16 +168,11 @@ pub async fn list(pool: &PgPool, query: &ListQuery) -> AppResult<(Vec<Manga>, Op
         SortField::Created => "created_at",
         SortField::Updated => "updated_at",
         SortField::Title => "lower(title)",
-        // Sorts on the alphabetically-first attached author. As an ORDER BY
-        // key this correlated subquery is evaluated per filter-matching row
-        // before LIMIT applies, so it scales worse than the indexed date/title
-        // sorts — revisit with a LATERAL join or precomputed sort-name column
-        // if the library grows large.
-        SortField::Author => {
-            "(SELECT min(lower(a.name)) \
-               FROM manga_authors ma JOIN authors a ON a.id = ma.author_id \
-              WHERE ma.manga_id = mangas.id)"
-        }
+        // Sorts on the alphabetically-first attached author. Precomputed into
+        // `mangas.sort_author` (migration 0037, maintained by triggers on
+        // manga_authors) and index-backed by `mangas_sort_author_idx`, so this
+        // is a plain column read rather than a per-row correlated subquery.
+        SortField::Author => "sort_author",
     };
     let dir = match query.order {
         SortOrder::Asc => "ASC",