auth.config.ts holds only the Edge-Runtime-safe parts of the Auth.js
config (session strategy, pages, jwt/session callbacks) with an empty
providers array. auth.ts spreads it and adds the real Credentials
provider on top for use in API routes/Server Components (Node runtime).
Needed for middleware.ts (next commit): the Credentials provider's
authorize() pulls in bcryptjs via verifyUserCredentials(), which is a
Node API not supported on the Edge Runtime that middleware.ts runs on.
Without this split, next build bundles bcryptjs/jose into the Edge
middleware chunk and warns; with it, the build is clean.
