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/migrations/0037_mangas_sort_author.sql

@@ -0,0 +1,51 @@
+-- Precomputed author sort key for `?sort=author`.
+--
+-- The author sort used a correlated `min(lower(a.name))` subquery as the ORDER
+-- BY key, evaluated per filter-matching row before LIMIT — it scaled worse than
+-- the indexed date/title sorts. Materialize the same value on `mangas` so the
+-- sort is a plain indexed column read.
+--
+-- `sort_author` = the alphabetically-first attached author's lowercased name,
+-- or NULL when the manga has no authors (kept last via NULLS LAST in the query).
+-- Author names are immutable (authors are upserted by unique lowercased name and
+-- never renamed), so the value only changes when the manga_authors join changes
+-- — maintained by the triggers below.
+
+ALTER TABLE mangas ADD COLUMN sort_author text;
+
+CREATE INDEX mangas_sort_author_idx ON mangas (sort_author, id);
+
+-- Backfill from existing links.
+UPDATE mangas m
+   SET sort_author = (
+        SELECT min(lower(a.name))
+          FROM manga_authors ma
+          JOIN authors a ON a.id = ma.author_id
+         WHERE ma.manga_id = m.id
+   );
+
+CREATE OR REPLACE FUNCTION refresh_manga_sort_author(mid uuid) RETURNS void AS $$
+BEGIN
+    UPDATE mangas
+       SET sort_author = (
+            SELECT min(lower(a.name))
+              FROM manga_authors ma
+              JOIN authors a ON a.id = ma.author_id
+             WHERE ma.manga_id = mid
+       )
+     WHERE id = mid;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE FUNCTION mangas_sort_author_on_ma_change() RETURNS trigger AS $$
+BEGIN
+    -- On a manga cascade-delete the UPDATE simply no-ops (row already gone).
+    PERFORM refresh_manga_sort_author(COALESCE(NEW.manga_id, OLD.manga_id));
+    RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER manga_authors_sort_author_ins AFTER INSERT ON manga_authors
+    FOR EACH ROW EXECUTE FUNCTION mangas_sort_author_on_ma_change();
+CREATE TRIGGER manga_authors_sort_author_del AFTER DELETE ON manga_authors
+    FOR EACH ROW EXECUTE FUNCTION mangas_sort_author_on_ma_change();