feat(telegram): promo bot + channel-chat moderation gate

Add a second standalone promo bot to the bot container (answers /start with a
localized message + a URL button into the main bot's Mini App) and gate write
access in a channel's linked discussion chat: grant on join when the Telegram
user is registered and neither admin-suspended nor holding a new chat_muted
role, and revoke/grant on the matching moderation change for a member currently
in the chat.

Eligibility (registered AND NOT suspended AND NOT chat_muted; the game
suspension dominates) is resolved once in the backend and reached two ways: the
bot's join-time unary ResolveChatEligibility over the existing mTLS bot-link,
and a backend chat_access_changed event -> gateway -> ChatGate command
(idempotent; a temporary-block-expiry sweeper may over-emit). The bot guards the
block/unblock path with getChatMember, since bots cannot list members.

A web_app button cannot open another bot's Mini App (it signs initData with the
sending bot's token), so the promo button is a t.me ?startapp URL reusing the
UI's VITE_TELEGRAM_LINK. The bot must be a chat admin with the restrict-members
right and chat_member in its allowed updates.

No schema change: chat_muted reuses the data-driven account_roles table.

gateway/internal/botlink/hub_test.go

@@ -8,7 +8,9 @@ import (
 	"time"
 
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/status"
 	"google.golang.org/grpc/test/bufconn"
 
 	botlinkv1 "scrabble/pkg/proto/botlink/v1"
@@ -23,12 +25,18 @@ type fakeBot struct {
 	received  chan *botlinkv1.Command
 }
 
-// startHub registers a Hub on an in-memory gRPC server and returns the hub plus a
-// dialer for fake bots.
+// startHub registers a Hub (no chat-eligibility resolver) on an in-memory gRPC
+// server and returns the hub plus a dialer for fake bots.
 func startHub(t *testing.T) (*Hub, func(t *testing.T) botlinkv1.BotLinkClient) {
+	return startHubWith(t, nil)
+}
+
+// startHubWith is startHub with an explicit chat-eligibility resolver, for the
+// ResolveChatEligibility tests.
+func startHubWith(t *testing.T, resolve EligibilityResolver) (*Hub, func(t *testing.T) botlinkv1.BotLinkClient) {
 	t.Helper()
 	lis := bufconn.Listen(1 << 20)
-	hub := NewHub(nil, nil)
+	hub := NewHub(nil, nil, resolve)
 	srv := grpc.NewServer()
 	botlinkv1.RegisterBotLinkServer(srv, hub)
 	go func() { _ = srv.Serve(lis) }()
@@ -162,6 +170,62 @@ func TestHubSendAsync(t *testing.T) {
 	}
 }
 
+func TestHubSendChatGate(t *testing.T) {
+	hub, dial := startHub(t)
+	ctx := t.Context()
+	bot := &fakeBot{ack: false}
+	bot.connect(t, ctx, hub, dial(t))
+
+	hub.Send(ChatGateCommand("42", true))
+	select {
+	case cmd := <-bot.received:
+		cg := cmd.GetChatGate()
+		if cg.GetExternalId() != "42" || !cg.GetAllow() {
+			t.Errorf("chat_gate = %+v, want external_id=42 allow=true", cg)
+		}
+	case <-time.After(time.Second):
+		t.Fatal("bot received no command")
+	}
+}
+
+func TestHubResolveChatEligibility(t *testing.T) {
+	var gotExt string
+	_, dial := startHubWith(t, func(_ context.Context, ext string) (bool, bool, error) {
+		gotExt = ext
+		return true, ext == "good", nil
+	})
+	client := dial(t)
+	ctx := t.Context()
+
+	resp, err := client.ResolveChatEligibility(ctx, &botlinkv1.ChatEligibilityRequest{ExternalId: "good"})
+	if err != nil {
+		t.Fatalf("ResolveChatEligibility: %v", err)
+	}
+	if gotExt != "good" {
+		t.Errorf("resolver external_id = %q, want good", gotExt)
+	}
+	if !resp.GetRegistered() || !resp.GetEligible() {
+		t.Errorf("resp = %+v, want registered+eligible", resp)
+	}
+
+	resp, err = client.ResolveChatEligibility(ctx, &botlinkv1.ChatEligibilityRequest{ExternalId: "muted"})
+	if err != nil {
+		t.Fatalf("ResolveChatEligibility(muted): %v", err)
+	}
+	if !resp.GetRegistered() || resp.GetEligible() {
+		t.Errorf("resp = %+v, want registered but not eligible", resp)
+	}
+}
+
+func TestHubResolveChatEligibilityUnconfigured(t *testing.T) {
+	_, dial := startHub(t) // nil resolver
+	client := dial(t)
+	_, err := client.ResolveChatEligibility(t.Context(), &botlinkv1.ChatEligibilityRequest{ExternalId: "x"})
+	if status.Code(err) != codes.Unavailable {
+		t.Fatalf("err = %v, want Unavailable", err)
+	}
+}
+
 func TestRelayServerNoBot(t *testing.T) {
 	hub, _ := startHub(t)
 	relay := NewRelayServer(hub, 200*time.Millisecond)