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/session/cache_test.go

@@ -0,0 +1,74 @@
+package session
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+type fakeResolver struct {
+	uid   string
+	err   error
+	calls int
+}
+
+func (f *fakeResolver) ResolveSession(_ context.Context, _ string) (string, error) {
+	f.calls++
+	if f.err != nil {
+		return "", f.err
+	}
+	return f.uid, nil
+}
+
+func TestResolveCachesBackendHit(t *testing.T) {
+	r := &fakeResolver{uid: "user-1"}
+	c := NewCache(r, time.Minute, 10)
+
+	for i := 0; i < 3; i++ {
+		uid, err := c.Resolve(context.Background(), "tok")
+		if err != nil || uid != "user-1" {
+			t.Fatalf("resolve #%d = (%q, %v)", i, uid, err)
+		}
+	}
+	if r.calls != 1 {
+		t.Fatalf("backend calls = %d, want 1 (cached)", r.calls)
+	}
+}
+
+func TestResolvePropagatesBackendError(t *testing.T) {
+	r := &fakeResolver{err: errors.New("nope")}
+	c := NewCache(r, time.Minute, 10)
+	if _, err := c.Resolve(context.Background(), "tok"); err == nil {
+		t.Fatal("expected backend error to propagate")
+	}
+}
+
+func TestResolveReResolvesAfterTTL(t *testing.T) {
+	r := &fakeResolver{uid: "user-1"}
+	c := NewCache(r, time.Minute, 10)
+	base := time.Now()
+	c.now = func() time.Time { return base }
+
+	if _, err := c.Resolve(context.Background(), "tok"); err != nil {
+		t.Fatal(err)
+	}
+	c.now = func() time.Time { return base.Add(2 * time.Minute) } // past TTL
+	if _, err := c.Resolve(context.Background(), "tok"); err != nil {
+		t.Fatal(err)
+	}
+	if r.calls != 2 {
+		t.Fatalf("backend calls = %d, want 2 (re-resolve after expiry)", r.calls)
+	}
+}
+
+func TestInvalidateForcesReResolve(t *testing.T) {
+	r := &fakeResolver{uid: "user-1"}
+	c := NewCache(r, time.Minute, 10)
+	_, _ = c.Resolve(context.Background(), "tok")
+	c.Invalidate("tok")
+	_, _ = c.Resolve(context.Background(), "tok")
+	if r.calls != 2 {
+		t.Fatalf("backend calls = %d, want 2 after invalidate", r.calls)
+	}
+}