Authorize a deployment request
Callback used by the messaging container — and by any agent that handles its own
HTTP requests (e.g. a frontend agent) — to check whether an inbound request should
be allowed against this deployment's grants.
Authentication is the per-deployment JWT injected as `ASTRO_AUTHZ_TOKEN`. The token's
`sub` claim identifies the deployment; no other deployment ID is passed in the request.
Returns `200` with the decision on every authoritative answer. Identity fields are
only populated when `allowed: true` — denials don't leak mapping state.