The 5-second startup-grace filter in _on_room_message silently drops
events where event_ts < startup_ts - 5. When the host clock is set
ahead of real time, the comparison flips against every live event and
the bot 'connects but never replies' — exactly the symptom in #12614.
Reporter Schnurzel700 chased this for several weeks before tracing it
to their Debian VM's clock being out of sync. The current /1000.0
millisecond->second conversion is correct (mautrix returns ms); the
failure mode is purely environmental.
Add a one-shot WARNING that fires when:
- we are >30s past startup (initial-sync replay window closed), AND
- 3 consecutive drops share the same skew within 60s (a constant
clock offset, not varied-age backfill from an invited room).
State is reset in connect() so reconnects after fixing NTP rearm the
detector. Includes the NTP fix instruction in the warning message
itself and a new Troubleshooting entry in the Matrix docs.
5 new tests cover the happy path, initial-sync backfill, under-
threshold drops, varied-age backfill, and the reconnect rearm path.
519657aa98