docs: updated to new auth challenge format and removed stale TOCTOU race condition note

server/crates/arbiter-server/migrations/2026-02-14-171124-0000_init/up.sql

@@ -71,7 +71,6 @@ create unique index if not exists uniq_metadata_binding_client on client_metadat
 
 create table if not exists program_client (
     id integer not null primary key,
-    nonce integer not null default(1), -- used for auth challenge
     public_key blob not null,
     metadata_id integer not null references client_metadata (id) on delete cascade,
     created_at integer not null default(unixepoch ('now')),