Login reached the backend (HTTP 200) but NextAuth returned CredentialsSignin.
Cause: authorize() read data.member.id/email/clubName/clubId, but the backend
LoginResponse is flat — { accessToken, refreshToken, expiresIn, role } with no
member object. Accessing data.member.id on undefined threw, so authorize()
returned null.
Decode the JWT payload to recover identity claims (sub=userId, email,
tenant_id=clubId) and use the flat top-level role. Adds a small decodeJwtPayload
helper (claims only, no signature verification needed here).