fix(hub): require exp, sub and a known scope when decoding access tokens

One Ed25519 key signs four kinds of token — user access, node access,
revocation broadcasts (no exp, no sub, and returned in the body of
POST /v1/admin/revoke and pushed to every node) and MHP federation tokens
(aud, sub=hub_id, no scope). decode_access_token required none of these, so a
hub-signed token with no exp was accepted and separation between the types
rested only on which fields each consumer happened to read.

Require exp, sub and scope, and reject a scope that is not one of the two
access scopes. A revocation or MHP token can no longer be mistaken for a
session, and no hub-signed token without an expiry is honoured.

A full RFC 5987 aud binding is deliberately not used: the node decodes its own
hub-issued token without passing audience, so adding aud would make every
already-deployed node reject its own token (InvalidAudienceError) — a
coordinated, node-breaking change. scope gives the same purpose separation
among the hub's own token types without it. This is non-breaking: every real
access token already carries exp, sub and scope, so no session is forced to
re-authenticate.

test_token_hardening.py holds the refusals (no exp, no scope, unknown scope, a
revocation token as bearer) and the paths that must keep working (a real login
token, a node token); verified red against the pre-fix auth.py and green after.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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