fix: the admin requests list names who asked, not a cut-off user id (#6265)

/admin/requests showed "by user f7e1ec86" — the first eight characters of
the requester's id, a placeholder from the first build. The admin list now
sends each requester's username, looked up for the whole page in one query,
and the row reads "by <username>". The id stays as the fallback for a list
that comes back without names.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/queries/users.sql

@@ -43,6 +43,11 @@ UPDATE users SET subsonic_password = $2 WHERE id = $1;
 -- name: GetUserByID :one
 SELECT * FROM users WHERE id = $1;
 
+-- name: ListUsernamesByIDs :many
+-- Names a list's people in one round trip, e.g. who made each request on
+-- /admin/requests.
+SELECT id, username FROM users WHERE id = ANY($1::uuid[]);
+
 -- name: SetListenBrainzToken :exec
 UPDATE users
 SET listenbrainz_token = $2,