news: tell Pete who's out there right now, not just who died

We only ever told Pete about outcomes. Nothing emitted when an expedition
*started*, which is why the two bored adventurers walked into dungeons and the
news feed said nothing at all — it wasn't broken, it had nothing to say.

Two halves:

A roster snapshot, pushed every 2 minutes. Deliberately NOT on the durable fact
queue: a fact is history and losing it loses it forever, so it retries. A
snapshot is a photograph of the present, and a retried one is a lie — by the
time it lands, she's moved. The next tick carries the truth. That's also what
lets Pete's staleness timer work: if we stay down, nothing arrives, and the
board stops claiming to be live instead of insisting forever that Josie is
still in holymachina.

And a "departure" bulletin when a bored adventurer lets itself out.

The snapshot omits opted-out players rather than anonymizing them, and carries a
board token distinct from every event token, so a standing row can't become the
key that links a player's dispatches back together.

The player_meta scan folds last_player_action_at/created_at in Go instead of
COALESCE()ing in SQL — modernc rebuilds time.Time from the declared column type
and COALESCE erases it. A failed scan here would publish an empty board and
every adventurer would vanish from the page.

internal/plugin/expedition_boredom.go

@@ -286,6 +286,7 @@ func (p *AdventurePlugin) tryBoredomStart(uid id.UserID, now time.Time) error {
 
 	slog.Info("boredom: adventurer left on its own",
 		"user", uid, "zone", zone.ID, "level", c.Level, "cost", int(cost))
+	emitBoredomDeparture(uid, zone, c.Level)
 	return p.SendDM(uid, renderBoredomDepartureDM(zone, supplies, purchase))
 }