CircleService.js: fetches the remote actor + outbox per circle_link,
verifies the Ed25519 Klonkt-Signature, sanitises (HTML -> plain text),
caches in remote_actors/remote_posts. Defensive: https-only, fetch timeout,
body cap, max items. Security: same-origin object.id vs actor (anti-impersonation),
TOFU on the public key (key change = re-confirmation required), pruning of
disappeared posts.