Adds an additive bearer-verification path: verify RS256 access tokens against
Tessera's JWKS (iss/aud/exp), map sub/preferred_username/email + roles
(realm_access.roles, resource_access.<audience>.roles) to the app's identity.
Existing auth (API keys / app JWTs / sessions) is unchanged. Issuer + audience
are env-configurable. Validated end-to-end against the local sim.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
