feat(db): add GetBooksByTag query for tag detail page

Uses $2 = ANY(tags) to match against the tags text[] column with GIN
index support. sqlc generates a single string Column2 param (not []string).

internal/database/queries/queries.sql

@@ -2167,3 +2167,8 @@ SELECT
 FROM libraries l
 JOIN library_types lt ON l.library_type_id = lt.id
 WHERE l.id = $1;
+
+-- name: GetBooksByTag :many
+SELECT * FROM media_items
+WHERE library_id = $1 AND tags @> ARRAY[$2::text]
+ORDER BY title ASC;