Stage 6: gateway edge (Connect/FlatBuffers over h2c, platform/email/guest auth, sessions, rate-limit, admin passthrough, live push bridge)

New public ingress and the first network edge. Framework + a vertical slice of
operations end-to-end; remaining ops reuse the same transcode pattern in Stage 7.

Contracts (new module scrabble/pkg):
- push.proto (backend->gateway gRPC server-stream) + scrabble.fbs (FlatBuffers
  edge payloads), committed generated Go; buf/flatc Makefiles (dev-time codegen).

Backend:
- REST handlers on the /api/v1 groups: internal session endpoints
  (telegram/guest/email login -> mint, resolve, revoke) and the user slice
  (profile, submit_play, state, lobby enqueue/poll, chat).
- internal/notify in-process Publisher hub + internal/pushgrpc gRPC server
  (BACKEND_GRPC_ADDR) streaming your_turn/opponent_moved/chat/nudge/match_found;
  emission in game.commit, social, matchmaker.
- migration 00005 accounts.is_guest; guests are durable rows excluded from stats;
  ProvisionGuest; email-as-login (RequestLoginCode/LoginWithCode).

Gateway (new module scrabble/gateway):
- Connect Gateway service over h2c (Execute + Subscribe), FlatBuffers<->JSON
  transcode registry, Telegram initData HMAC validator (seam), session cache,
  token-bucket rate limiter (3 classes), push fan-out hub, backend REST + push
  gRPC client, admin Basic-Auth reverse proxy.

go.work: use ./pkg, ./gateway + replace scrabble/pkg. CI: gateway/**, pkg/**
path filters; unit build/vet/test span all three modules. Docs (PLAN,
ARCHITECTURE, FUNCTIONAL+ru, TESTING, READMEs) updated; gateway/pkg unit tests +
guest/email-login integration tests.

gateway/internal/push/hub_test.go

@@ -0,0 +1,56 @@
+package push_test
+
+import (
+	"testing"
+
+	"scrabble/gateway/internal/push"
+)
+
+func TestHubRoutesByUser(t *testing.T) {
+	h := push.NewHub(4)
+	chA, cancelA := h.Subscribe("user-a")
+	defer cancelA()
+	chB, cancelB := h.Subscribe("user-b")
+	defer cancelB()
+
+	h.Publish(push.Event{UserID: "user-a", Kind: "your_turn"})
+
+	select {
+	case e := <-chA:
+		if e.Kind != "your_turn" {
+			t.Fatalf("user-a received %q", e.Kind)
+		}
+	default:
+		t.Fatal("user-a should have received the event")
+	}
+	select {
+	case <-chB:
+		t.Fatal("user-b must not receive user-a's event")
+	default:
+	}
+}
+
+func TestHubDropsOnOverflow(t *testing.T) {
+	h := push.NewHub(1)
+	ch, cancel := h.Subscribe("u")
+	defer cancel()
+	for i := 0; i < 5; i++ {
+		h.Publish(push.Event{UserID: "u", Kind: "chat_message"})
+	}
+	if got := len(ch); got != 1 {
+		t.Fatalf("buffered %d events, want 1 (overflow dropped)", got)
+	}
+}
+
+func TestHubUnsubscribeClosesChannel(t *testing.T) {
+	h := push.NewHub(2)
+	ch, cancel := h.Subscribe("u")
+	cancel()
+	if _, ok := <-ch; ok {
+		t.Fatal("channel should be closed after unsubscribe")
+	}
+	if h.SubscriberCount() != 0 {
+		t.Fatalf("subscriber count = %d, want 0", h.SubscriberCount())
+	}
+	h.Publish(push.Event{UserID: "u"}) // must not panic
+}