Reject disposable / throwaway email providers (mailinator, yopmail,
10minutemail, ...) across all signup paths: email/password, Google,
Microsoft, personal OIDC and organisation OIDC. Backed by the
mailchecker package (offline, ~55k domains, subdomain-aware).
Exposes a SIGNUP_DISPOSABLE_EMAIL error code so the signup form and
SSO redirect alert can show a dedicated message instead of the
generic 'signup disabled' one.