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.

platform/telegram/internal/botlink/executor_test.go

@@ -13,9 +13,11 @@ import (
 
 // fakeSender records the delivery calls the executor makes.
 type fakeSender struct {
-	notify []notifyCall
-	text   []textCall
-	err    error
+	notify  []notifyCall
+	text    []textCall
+	gate    []gateCall
+	applied bool // ApplyChatGate's reported result
+	err     error
 }
 
 type notifyCall struct {
@@ -26,6 +28,10 @@ type textCall struct {
 	chatID int64
 	text   string
 }
+type gateCall struct {
+	userID int64
+	allow  bool
+}
 
 func (f *fakeSender) Notify(_ context.Context, chatID int64, text, buttonText, startParam string) error {
 	f.notify = append(f.notify, notifyCall{chatID, text, buttonText, startParam})
@@ -37,6 +43,11 @@ func (f *fakeSender) SendText(_ context.Context, chatID int64, text string) erro
 	return f.err
 }
 
+func (f *fakeSender) ApplyChatGate(_ context.Context, userID int64, allow bool) (bool, error) {
+	f.gate = append(f.gate, gateCall{userID, allow})
+	return f.applied, f.err
+}
+
 func yourTurnPayload(gameID string) []byte {
 	b := flatbuffers.NewBuilder(0)
 	gid := b.CreateString(gameID)
@@ -106,6 +117,46 @@ func TestExecutorSendToUser(t *testing.T) {
 	}
 }
 
+func chatGateCmd(externalID string, allow bool) *botlinkv1.Command {
+	return &botlinkv1.Command{Payload: &botlinkv1.Command_ChatGate{ChatGate: &botlinkv1.ChatGateCommand{
+		ExternalId: externalID, Allow: allow,
+	}}}
+}
+
+func TestExecutorChatGateApplied(t *testing.T) {
+	sender := &fakeSender{applied: true}
+	exec := NewExecutor(sender, 0, nil)
+	delivered, err := exec.Handle(context.Background(), chatGateCmd("777", true))
+	if err != nil {
+		t.Fatalf("handle: %v", err)
+	}
+	if !delivered || len(sender.gate) != 1 || sender.gate[0].userID != 777 || !sender.gate[0].allow {
+		t.Errorf("chat gate = %v / calls %+v", delivered, sender.gate)
+	}
+}
+
+func TestExecutorChatGateNotInChat(t *testing.T) {
+	sender := &fakeSender{applied: false} // user not in the chat
+	exec := NewExecutor(sender, 0, nil)
+	delivered, err := exec.Handle(context.Background(), chatGateCmd("888", false))
+	if err != nil {
+		t.Fatalf("handle: %v", err)
+	}
+	if delivered {
+		t.Error("expected delivered=false when the user is not in the chat")
+	}
+	if len(sender.gate) != 1 {
+		t.Errorf("gate calls = %d, want 1", len(sender.gate))
+	}
+}
+
+func TestExecutorChatGateInvalidExternalID(t *testing.T) {
+	exec := NewExecutor(&fakeSender{}, 0, nil)
+	if _, err := exec.Handle(context.Background(), chatGateCmd("not-a-number", true)); err == nil {
+		t.Error("expected an error for a non-numeric external_id")
+	}
+}
+
 func TestExecutorSendToChannel(t *testing.T) {
 	channelCmd := &botlinkv1.Command{Payload: &botlinkv1.Command_SendToChannel{SendToChannel: &telegramv1.SendToGameChannelRequest{Text: "news"}}}