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/auth/telegram_test.go

@@ -0,0 +1,92 @@
+package auth_test
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
+	"net/url"
+	"sort"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"scrabble/gateway/internal/auth"
+)
+
+// signedInitData builds a valid Telegram initData query string for botToken,
+// computing the hash exactly as Telegram does.
+func signedInitData(botToken string, fields map[string]string) string {
+	keys := make([]string, 0, len(fields))
+	for k := range fields {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	lines := make([]string, 0, len(keys))
+	for _, k := range keys {
+		lines = append(lines, k+"="+fields[k])
+	}
+	secretMAC := hmac.New(sha256.New, []byte("WebAppData"))
+	secretMAC.Write([]byte(botToken))
+	secret := secretMAC.Sum(nil)
+	mac := hmac.New(sha256.New, secret)
+	mac.Write([]byte(strings.Join(lines, "\n")))
+	hash := hex.EncodeToString(mac.Sum(nil))
+
+	v := url.Values{}
+	for k, val := range fields {
+		v.Set(k, val)
+	}
+	v.Set("hash", hash)
+	return v.Encode()
+}
+
+func TestValidateAcceptsGenuineInitData(t *testing.T) {
+	const token = "test-bot-token"
+	fields := map[string]string{
+		"auth_date": strconv.FormatInt(time.Now().Unix(), 10),
+		"query_id":  "abc",
+		"user":      `{"id":42,"first_name":"Ann","username":"ann"}`,
+	}
+	u, err := auth.NewHMACValidator(token).Validate(signedInitData(token, fields))
+	if err != nil {
+		t.Fatalf("validate genuine: %v", err)
+	}
+	if u.ID != "42" || u.Username != "ann" {
+		t.Fatalf("user = %+v", u)
+	}
+}
+
+func TestValidateRejectsTamperedHash(t *testing.T) {
+	const token = "test-bot-token"
+	fields := map[string]string{
+		"auth_date": strconv.FormatInt(time.Now().Unix(), 10),
+		"user":      `{"id":42}`,
+	}
+	data := signedInitData(token, fields) + "0" // corrupt the trailing hash
+	if _, err := auth.NewHMACValidator(token).Validate(data); err == nil {
+		t.Fatal("expected rejection of tampered init data")
+	}
+}
+
+func TestValidateRejectsWrongToken(t *testing.T) {
+	fields := map[string]string{
+		"auth_date": strconv.FormatInt(time.Now().Unix(), 10),
+		"user":      `{"id":42}`,
+	}
+	data := signedInitData("real-token", fields)
+	if _, err := auth.NewHMACValidator("other-token").Validate(data); err == nil {
+		t.Fatal("expected rejection under a different bot token")
+	}
+}
+
+func TestValidateRejectsStaleInitData(t *testing.T) {
+	const token = "test-bot-token"
+	fields := map[string]string{
+		"auth_date": strconv.FormatInt(time.Now().Add(-48*time.Hour).Unix(), 10),
+		"user":      `{"id":42}`,
+	}
+	if _, err := auth.NewHMACValidator(token).Validate(signedInitData(token, fields)); err == nil {
+		t.Fatal("expected rejection of stale init data")
+	}
+}