The one client-facing surface for ready checks.

A player holds at most one open check per lane — the match lane (lobby ready-up or matchmaking accept) and the party lane (the party's standing board) — so answering needs no id, just a scope. GET /me/ready_check returns both lanes; POST /me/ready_check answers one of them.

POST /lobbies/ready_check and POST /parties/ready_check are reset semantics: they quietly replace any open board with a fresh one over the current members, so the same endpoint serves "open", "force ready" (pass a timeout_ms) and "start over".