feat: block disposable email signups (#2883)

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.

packages/auth/server/lib/errors/error-codes.ts

@@ -18,6 +18,7 @@ export const AuthenticationErrorCode = {
   // TwoFactorMissingCredentials: 'TWO_FACTOR_MISSING_CREDENTIALS',
   InvalidTwoFactorCode: 'INVALID_TWO_FACTOR_CODE',
   SignupDisabled: 'SIGNUP_DISABLED',
+  SignupDisposableEmail: 'SIGNUP_DISPOSABLE_EMAIL',
   // IncorrectTwoFactorBackupCode: 'INCORRECT_TWO_FACTOR_BACKUP_CODE',
   // IncorrectIdentityProvider: 'INCORRECT_IDENTITY_PROVIDER',
   // IncorrectPassword: 'INCORRECT_PASSWORD',