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_setup.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"gogobee/internal/db"
+	"gogobee/internal/peteclient"
 	"maunium.net/go/mautrix/id"
 )
 
@@ -290,6 +291,22 @@ func (p *AdventurePlugin) dndSetupConfirm(ctx MessageContext) error {
 	// Idempotent — !setup confirm after a respec wipe will repopulate.
 	_ = ensureSpellsForCharacter(c)
 
+	// A new character walked through the gates — file a welcome dispatch to
+	// Pete. GUID keyed on the user alone so a later respec/re-confirm can't
+	// re-announce the same arrival. No-op unless the seam is enabled.
+	if name := charName(ctx.Sender); name != "" {
+		ts := nowUnix()
+		emitFact(peteclient.Fact{
+			GUID:       "arrival:" + userHash(ctx.Sender),
+			EventType:  "arrival",
+			Tier:       "bulletin",
+			Subject:    name,
+			ClassRace:  classRaceLabel(c),
+			Level:      c.Level,
+			OccurredAt: ts,
+		}, ctx.Sender, "")
+	}
+
 	return p.SendDM(ctx.Sender, renderSetupComplete(c))
 }