Resolving a session cookie failed identically whether the credential was
missing or Postgres was unreachable: local_auth_enabled read errors fell into
the fail-closed 'disabled' branch and SessionUser errors into 'invalid
session', both surfacing as 401 'authentication required' — a lie that reads
as 'log in again' during an outage. Split the enabled-read into
(enabled, error), tag non-ErrNotFound store failures with errAuthBackend, and
map that to a new 503 auth_unavailable in requireExternal. Fail-closed is
unchanged: missing setting / bad value / missing session stay 401.
Lemon-miaow
2a8f897e61