perf(postgres): use SELECT 1 for healthcheck instead of COUNT on AcmeChallenge

`checkConnnectionStatus` ran `SELECT COUNT(domain) FROM "AcmeChallenge"`
on every status probe — k8s liveness, Prometheus scrape, /status hit —
all of which call this. The COUNT grows with table size and holds a
connection for no diagnostic benefit beyond what `SELECT 1` provides.