The message listener runs synchronously before the 200 ack, so a slow
delete+repost can blow Slack's 3s window and be retried with no guard.
Ignore retried deliveries (X-Slack-Retry-Num), stamp last_bump_ts before
the delete/repost so a mid-bump retry is debounced, and skip thread
replies that don't move the main timeline.
Refs: #135
Replace the native pin with true bottom-of-channel stickiness: a
debounced message.channels handler in the slack-bot Lambda deletes and
re-posts the tracked schedule message on new activity so it stays last,
while the Monday rollover and in-week edits keep editing the same stored
ts. The pin only gave header reachability, not bottom placement.
Refs: #135
