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/ratelimit/ratelimit_test.go

@@ -0,0 +1,46 @@
+package ratelimit_test
+
+import (
+	"testing"
+	"time"
+
+	"scrabble/gateway/internal/ratelimit"
+)
+
+func TestAllowEnforcesBurst(t *testing.T) {
+	l := ratelimit.New()
+	p := ratelimit.PerMinute(60, 3) // 1/s, burst 3
+	allowed := 0
+	for i := 0; i < 5; i++ {
+		if l.Allow("ip:1.2.3.4", p) {
+			allowed++
+		}
+	}
+	if allowed != 3 {
+		t.Fatalf("allowed %d of 5, want 3 (burst)", allowed)
+	}
+}
+
+func TestAllowIsolatesKeys(t *testing.T) {
+	l := ratelimit.New()
+	p := ratelimit.PerMinute(60, 1)
+	if !l.Allow("user:a", p) {
+		t.Fatal("first key should be allowed")
+	}
+	if !l.Allow("user:b", p) {
+		t.Fatal("a different key must have its own bucket")
+	}
+	if l.Allow("user:a", p) {
+		t.Fatal("the first key's bucket should now be empty")
+	}
+}
+
+func TestPerWindow(t *testing.T) {
+	// 5 events per 10 minutes, burst 2: the third immediate call is denied.
+	p := ratelimit.Per(5, 10*time.Minute, 2)
+	l := ratelimit.New()
+	got := []bool{l.Allow("email:x", p), l.Allow("email:x", p), l.Allow("email:x", p)}
+	if !got[0] || !got[1] || got[2] {
+		t.Fatalf("per-window burst = %v, want [true true false]", got)
+	}
+}