After shutdown, the replenisher stops adding permits but signers are
blocked forever in acquire_rate_permit().await. Call limiter.close() so
pending acquire_owned() calls return AcquireError, which the signer
already handles gracefully by breaking out of its loop.

Without this, all signers with target_tps configured deadlock on
shutdown, waiting for permits that will never come.

fix(gobrr): close rate limiter semaphore on shutdown