Every SSE client polled COUNT(*) on a fresh EventStore connection each ~1s
(re-applying the 473-line schema unless PG_SKIP_SCHEMA=1). Replaced with one
process-wide _ChangeSignal: a single long-lived connection reads max(timestamp)
(idx_events_timestamp tail seek) at most once per second GLOBALLY behind a lock,
so N clients cost one query/sec. EventStore.max_timestamp() backs it; reader is
injectable for the query-budget test.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>