Fixes#533, found while working on #529/#531.
OIDC and SAML logins hard-blocked on the MFA policy with zero check of whether the user actually enrolled TOTP, and no challenge step at all. Once an admin turned on an MFA-required policy, every SSO user was permanently locked out regardless of enrollment status.
- Extract the post-auth MFA decision (challenge / enrollment-required / proceed) into a shared, unit-tested function so OIDC and SAML can't independently diverge again
- An already-enrolled user now gets a real challenge (reusing the existing, auth-method-agnostic MFA completion flow) instead of being blocked
- An unenrolled user under a required policy gets a distinct, correctly mapped error instead of the old generic one
- Fix a real fail-open regression caught in review: a transient DB error during the enrollment-status check could have silently skipped MFA entirely for an enrolled user; now it fails closed and logs
- Strip the one-time challenge token from the URL after consuming it
