* fix(filer.sync): keep sync_offset fresh while the source is read-only
sync_offset holds the timestamp of the last replicated source event, so
monitoring derives lag from now-sync_offset. A read-only source emits no
metadata events, so the gauge froze at the last write and the derived lag
grew without bound, making thresholds unusable.
The source filer now sends an idle heartbeat carrying its current time
while a subscriber is caught up to the buffer head. filer.sync uses it to
advance the gauge, so now-sync_offset reflects real lag. Heartbeats are
opt-in (client_supports_idle_heartbeat), are never written to the metadata
log, and do not move the resume checkpoint, so a restart still resumes
from the last real event.
* fix(filer.sync): gate idle heartbeat on the read cursor, not SinceNs
In metadata-chunks mode persisted entries replay as log file refs and
never reach eachLogEntryFn, so lastSeenTsNs stays put and a caught-up
subscriber with an old SinceNs would never get a heartbeat. Use the
read cursor (lastReadTime), which advances in that mode too, max'd with
lastSeenTsNs so the in-memory backlog-then-idle case still works while
the cursor returned to the caller has not yet updated.
5af7d12f04