ADR-011's single-user authz (ID-token subject must equal AllowedSubject,
else 403) is replaced by ADR-012's model: Dex authentication is the only
gate — any Dex-authenticated subject may establish a session. Whether that
subject has a tapir user, and routing to registration if not, is decided
downstream in internal/web (next commit).
Removals (noted): oidc.Config.AllowedSubject + its required-field check + the
callback 403 branch; config.Config.AllowedSubject + TAPIR_ALLOWED_SUBJECT env
wiring; the AllowedSubject arg in cmdServe. ui-spec.md updated to reflect the
supersession. Sessions, cookie signing, login/callback/logout unchanged.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>