Pete Adventure News: emit seam, !news command, cold-start backfill

gogobee's game-side of Pete's push-based Adventure news feed. Emits
structured event *facts* (not prose) to Pete's ingest endpoint over a
durable queue; Pete owns voice/authoring/publishing.

- internal/peteclient: durable pete_emit_queue + retry sender; Fact
  payload; FEATURE_PETE_NEWS master switch.
- pete.go: emitFact enforces runtime kill-switch + per-player opt-out
  (anonymize, never drop); zone-clear + realm-first tiering; !news
  command (player optout/optin, admin on/off).
- bootstrap_pete_news.go: cold-start backfill replays deaths +
  single-holder achievements + zone realm-firsts, backdated + NoPush,
  idempotent, fires only once the seam is live; seeds news_realm_firsts.
- Emit sites: death, zone_first, rival_result, milestone, arrival.
- db.go: pete_emit_queue, news_optout (+opted_out_at), news_realm_firsts.

Unshipped. Deploy Pete first, then set FEATURE_PETE_NEWS + ingest env.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/dnd_expedition_extract.go

@@ -177,6 +177,9 @@ func (p *AdventurePlugin) finalizeExpeditionOnZoneClear(userID id.UserID, runID
 	if sl := p.shadowOnPlayerZoneClear(userID, exp.ZoneID); sl != "" {
 		lines = append(lines, sl)
 	}
+	// News: boss down means the zone is cleared. Realm-first → PRIORITY,
+	// repeat → BULLETIN. No-op unless the Pete seam is enabled.
+	emitZoneClearNews(userID, exp)
 	return lines
 }