#4342 Shorten swamp serve's token session re-check so revocation takes effect sooner

Follow-up split from swamp-club#4340.

swamp serve re-checks open token sessions every 30 seconds (DEFAULT_TOKEN_SESSION_REVALIDATION_MS in src/serve/token_session_revalidation_service.ts). A collective token revoked in swamp-club (including the cascade when a minting token is revoked, swamp-club#4266) keeps its open serve sessions for up to that interval.

Ask

Decide on a shorter default interval, or a way to push revocations to serve, and whether the interval should be configurable.

Trade-off

A shorter interval narrows the revocation window but costs one swamp-club round trip per open token, per interval, per serve instance, and makes serve more sensitive to swamp-club latency and outages (a failed pass keeps sessions until the next one). A push or invalidation signal avoids the polling cost but needs a server-side channel.