feat(telegram): localized /start welcome with channel & chat follow links

The main bot answered /start with a single English line ("Tap to open Scrabble.").
Localize it: Russian or English by the sender's reported Telegram language
(Message.from.language_code, which the Bot API carries on the message itself — there is
no separate user-update event — English fallback), with the longer welcome copy and a
localized launch button ("Открыть «Эрудит»" / "Open “Erudite”").

The welcome links the game channel and the discussion chat by their public @username,
resolved once at startup from the configured TELEGRAM_GAME_CHANNEL_ID / TELEGRAM_CHAT_ID
via getChat and cached. A handle that is unset, private, or unreadable degrades to a
generic noun ("the channel" / "our chat") rather than a dangling "@", so the paragraph
always reads cleanly (the bot's info screen still lists the real links). Adds
GameChannelID to bot.Config (wired from the existing config) for the channel handle.

Tests: startText localization + handle embedding + per-slot generic fallback; handleStart
language selection; resolveWelcomeHandles. README updated.

platform/telegram/internal/bot/bot.go

@@ -33,8 +33,12 @@ type Config struct {
 	SendRatePerSecond int
 	// ChatID is the moderated discussion chat the bot gates write access in; 0
 	// disables chat gating (and the chat_member long-poll subscription). Gating needs
-	// the bot to be an administrator there with the restrict-members right.
+	// the bot to be an administrator there with the restrict-members right. Its public
+	// @username is also resolved at startup for the /start welcome's discussion link.
 	ChatID int64
+	// GameChannelID is the game channel whose public @username the /start welcome links
+	// to (resolved from this id via getChat at startup); 0 omits that follow link.
+	GameChannelID int64
 }
 
 // EligibilityResolver answers whether the Telegram user identified by externalID
@@ -54,6 +58,13 @@ type Bot struct {
 	limiter *rate.Limiter
 	// chatID is the moderated discussion chat (0 disables gating).
 	chatID int64
+	// channelID is the game channel (0 omits its welcome follow link).
+	channelID int64
+	// channelUsername and chatUsername are the public @usernames (without the leading
+	// @) of the game channel and the discussion chat, resolved once at startup
+	// (resolveWelcomeHandles) for the /start welcome's follow links; "" when unresolved.
+	channelUsername string
+	chatUsername    string
 	// botID is the bot's own Telegram user id (resolved at startup); it skips the
 	// chat_member updates the bot's own restrict actions generate — the grant loop guard.
 	botID int64
@@ -69,7 +80,7 @@ func New(cfg Config, log *zap.Logger) (*Bot, error) {
 	if log == nil {
 		log = zap.NewNop()
 	}
-	t := &Bot{miniAppURL: cfg.MiniAppURL, log: log, chatID: cfg.ChatID}
+	t := &Bot{miniAppURL: cfg.MiniAppURL, log: log, chatID: cfg.ChatID, channelID: cfg.GameChannelID}
 	if cfg.SendRatePerSecond > 0 {
 		t.limiter = rate.NewLimiter(rate.Limit(cfg.SendRatePerSecond), cfg.SendRatePerSecond)
 	}
@@ -123,9 +134,43 @@ func (t *Bot) Run(ctx context.Context) {
 	if t.chatID != 0 {
 		t.logChatAdminStatus(ctx)
 	}
+	t.resolveWelcomeHandles(ctx)
 	t.api.Start(ctx)
 }
 
+// resolveWelcomeHandles resolves, once at startup, the public @usernames of the game
+// channel and the discussion chat from their configured ids (getChat), caching them for
+// the /start welcome's follow links. It runs before the update loop, so the handles are
+// set before any /start is handled; a chat that is unset, private (no public username)
+// or unreadable simply leaves its handle empty and the welcome omits that follow link.
+func (t *Bot) resolveWelcomeHandles(ctx context.Context) {
+	t.channelUsername = t.resolveUsername(ctx, t.channelID, "game channel")
+	t.chatUsername = t.resolveUsername(ctx, t.chatID, "discussion chat")
+}
+
+// resolveUsername returns the public @username (without the leading @) of the chat with
+// the given id, or "" when id is 0, the chat has no public username, or getChat fails —
+// logging the reason, since a missing handle silently drops a welcome follow link.
+func (t *Bot) resolveUsername(ctx context.Context, id int64, label string) string {
+	if id == 0 {
+		return ""
+	}
+	chat, err := t.api.GetChat(ctx, &tgbot.GetChatParams{ChatID: id})
+	if err != nil {
+		t.log.Warn("welcome: getChat failed; follow link omitted",
+			zap.String("chat", label), zap.Int64("id", id), zap.Error(err))
+		return ""
+	}
+	if chat.Username == "" {
+		t.log.Warn("welcome: chat has no public @username; follow link omitted",
+			zap.String("chat", label), zap.Int64("id", id))
+		return ""
+	}
+	t.log.Info("welcome: resolved follow link",
+		zap.String("chat", label), zap.String("username", chat.Username))
+	return chat.Username
+}
+
 // logChatAdminStatus checks, at startup, whether the bot can actually gate the
 // moderated chat — it must be an administrator there with the restrict-members
 // ("Ban users") right, or Telegram delivers no chat_member updates and restricts
@@ -198,11 +243,19 @@ func (t *Bot) handleStart(ctx context.Context, api *tgbot.Bot, update *models.Up
 	if update.Message.Chat.Type != models.ChatTypePrivate {
 		return
 	}
+	// The sender's Telegram language rides on the message itself (Message.from.language_code
+	// in the Bot API — there is no separate user-update event); fall back to English when it
+	// is absent.
+	lang := ""
+	if update.Message.From != nil {
+		lang = update.Message.From.LanguageCode
+	}
+	text, button := startText(lang, t.channelUsername, t.chatUsername)
 	startParam := startPayload(update.Message.Text)
 	if _, err := api.SendMessage(ctx, &tgbot.SendMessageParams{
 		ChatID:      update.Message.Chat.ID,
-		Text:        "Tap to open Scrabble.",
-		ReplyMarkup: t.launchMarkup("Open Scrabble", startParam),
+		Text:        text,
+		ReplyMarkup: t.launchMarkup(button, startParam),
 	}); err != nil {
 		t.log.Warn("reply to start failed", zap.Error(err))
 	}

platform/telegram/internal/bot/bot_test.go

@@ -29,6 +29,10 @@ func (f *fakeBotAPI) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		f.text = r.FormValue("text")
 		f.replyMarkup = r.FormValue("reply_markup")
 		io.WriteString(w, `{"ok":true,"result":{"message_id":1}}`)
+	case strings.HasSuffix(r.URL.Path, "/getChat"):
+		// Echo the requested id into the username so a resolver test can tell the
+		// channel lookup from the chat lookup.
+		io.WriteString(w, `{"ok":true,"result":{"id":-100,"type":"channel","username":"u`+r.FormValue("chat_id")+`"}}`)
 	default:
 		io.WriteString(w, `{"ok":true,"result":true}`)
 	}
@@ -105,7 +109,7 @@ func TestTestEnvironmentRoutesGetMe(t *testing.T) {
 }
 
 func TestHandleStartRepliesPrivateOnly(t *testing.T) {
-	t.Run("private replies", func(t *testing.T) {
+	t.Run("private replies in english by default", func(t *testing.T) {
 		api := &fakeBotAPI{}
 		b := newTestBot(t, api)
 		b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
@@ -114,6 +118,35 @@ func TestHandleStartRepliesPrivateOnly(t *testing.T) {
 		if api.chatID != "42" || !strings.Contains(api.replyMarkup, "web_app") {
 			t.Errorf("private /start: chat=%q markup=%q, want a web_app reply", api.chatID, api.replyMarkup)
 		}
+		// No reported language -> English welcome + English button.
+		if !strings.Contains(api.text, "Hi!") {
+			t.Errorf("text = %q, want the English welcome", api.text)
+		}
+		if !strings.Contains(api.replyMarkup, "Open") {
+			t.Errorf("reply_markup = %q, want the English button", api.replyMarkup)
+		}
+	})
+	t.Run("uses the sender's reported language", func(t *testing.T) {
+		api := &fakeBotAPI{}
+		b := newTestBot(t, api)
+		b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
+			Chat: models.Chat{ID: 42, Type: models.ChatTypePrivate}, Text: "/start",
+			From: &models.User{ID: 7, LanguageCode: "ru"},
+		}})
+		if !strings.Contains(api.text, "Привет!") {
+			t.Errorf("text = %q, want the Russian welcome for a ru sender", api.text)
+		}
+	})
+	t.Run("embeds resolved follow handles", func(t *testing.T) {
+		api := &fakeBotAPI{}
+		b := newTestBot(t, api)
+		b.channelUsername, b.chatUsername = "erudit", "erudite_chat"
+		b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
+			Chat: models.Chat{ID: 42, Type: models.ChatTypePrivate}, Text: "/start",
+		}})
+		if !strings.Contains(api.text, "@erudit") || !strings.Contains(api.text, "@erudite_chat") {
+			t.Errorf("text = %q, want the follow handles", api.text)
+		}
 	})
 	t.Run("group ignored", func(t *testing.T) {
 		api := &fakeBotAPI{}
@@ -127,6 +160,26 @@ func TestHandleStartRepliesPrivateOnly(t *testing.T) {
 	})
 }
 
+func TestResolveWelcomeHandles(t *testing.T) {
+	api := &fakeBotAPI{}
+	b := newTestBot(t, api)
+	b.channelID, b.chatID = 111, 222
+	b.resolveWelcomeHandles(context.Background())
+	// The fake echoes the requested id into the username, so each lookup is independent.
+	if b.channelUsername != "u111" {
+		t.Errorf("channelUsername = %q, want u111", b.channelUsername)
+	}
+	if b.chatUsername != "u222" {
+		t.Errorf("chatUsername = %q, want u222", b.chatUsername)
+	}
+	// An unset id resolves to no handle (and makes no getChat call).
+	b.channelID = 0
+	b.resolveWelcomeHandles(context.Background())
+	if b.channelUsername != "" {
+		t.Errorf("channelUsername = %q, want empty for id 0", b.channelUsername)
+	}
+}
+
 func TestStartPayload(t *testing.T) {
 	cases := map[string]string{
 		"/start g123":  "g123",

platform/telegram/internal/bot/welcome.go

@@ -0,0 +1,60 @@
+package bot
+
+import "strings"
+
+// startText returns the localized /start welcome body and the launch-button label.
+// Russian is used when lang (the IETF language tag the Telegram client reports on the
+// message's sender) starts with "ru", English otherwise and when it is absent — so a
+// user with no reported language still gets a sensible message. channel and chat are
+// the resolved public @usernames (without the leading @) of the game channel and the
+// discussion chat; when either is empty its follow link degrades to a generic noun
+// (e.g. "the channel" / "our chat") rather than rendering a dangling "@", since the
+// bot's own info screen still lists the real links.
+func startText(lang, channel, chat string) (text, button string) {
+	if strings.HasPrefix(strings.ToLower(lang), "ru") {
+		return ruWelcome(channel, chat), "Открыть «Эрудит»"
+	}
+	return enWelcome(channel, chat), "Open “Erudite”"
+}
+
+// ruWelcome builds the Russian welcome. A known handle is named as "@<username>"; an
+// unresolved one degrades to a plain noun.
+func ruWelcome(channel, chat string) string {
+	ch := "канал"
+	if channel != "" {
+		ch = "@" + channel
+	}
+	ct := "чате"
+	if chat != "" {
+		ct = "@" + chat
+	}
+	return strings.Join([]string{
+		"Привет! 👋",
+		"Здесь можно сражаться в «Эрудит» со случайными игроками или в компании друзей.",
+		"Подписывайтесь на " + ch + ", чтобы быть в курсе последних игровых событий и вовремя " +
+			"получать важные уведомления. Игроки могут обсуждать игру и просто общаться в нашем " +
+			ct + "! 💬",
+		"Ни слова больше.\nПервая партия сама себя не сыграет 😊",
+	}, "\n\n")
+}
+
+// enWelcome builds the English welcome (the fallback for any non-Russian or missing
+// language). A known handle is named as "@<username>"; an unresolved one degrades to a
+// plain noun.
+func enWelcome(channel, chat string) string {
+	ch := "the channel"
+	if channel != "" {
+		ch = "@" + channel
+	}
+	ct := "group chat"
+	if chat != "" {
+		ct = "@" + chat
+	}
+	return strings.Join([]string{
+		"Hi! 👋",
+		"Play Scrabble against random players — or with a group of friends.",
+		"Follow " + ch + " to stay up to date with the latest game events and receive important " +
+			"notifications in time. Players can discuss the game and simply chat in our " + ct + "! 💬",
+		"Okay, no more talking.\nFirst game won't play itself 😊",
+	}, "\n\n")
+}

platform/telegram/internal/bot/welcome_test.go

@@ -0,0 +1,73 @@
+package bot
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestStartTextLocalizesByLanguage(t *testing.T) {
+	cases := []struct {
+		name       string
+		lang       string
+		wantButton string
+		wantSubstr string // a phrase unique to the chosen language body
+	}{
+		{"russian", "ru", "Открыть «Эрудит»", "Привет!"},
+		{"russian region tag", "ru-RU", "Открыть «Эрудит»", "Первая партия"},
+		{"english", "en", "Open “Erudite”", "Hi!"},
+		{"other language falls back to english", "de", "Open “Erudite”", "Hi!"},
+		{"absent language falls back to english", "", "Open “Erudite”", "no more talking"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			text, button := startText(tc.lang, "erudit", "erudite_chat")
+			if button != tc.wantButton {
+				t.Errorf("button = %q, want %q", button, tc.wantButton)
+			}
+			if !strings.Contains(text, tc.wantSubstr) {
+				t.Errorf("text %q does not contain %q", text, tc.wantSubstr)
+			}
+		})
+	}
+}
+
+func TestStartTextEmbedsFollowHandles(t *testing.T) {
+	for _, lang := range []string{"ru", "en"} {
+		text, _ := startText(lang, "erudit", "erudite_chat")
+		if !strings.Contains(text, "@erudit") || !strings.Contains(text, "@erudite_chat") {
+			t.Errorf("lang %q: follow paragraph missing the handles: %q", lang, text)
+		}
+	}
+}
+
+func TestStartTextFallsBackToGenericWhenHandleMissing(t *testing.T) {
+	// An unresolved handle degrades to a generic noun rather than a dangling "@" — and
+	// only that slot degrades; a resolved sibling still shows its "@username".
+	t.Run("both missing leaves no @", func(t *testing.T) {
+		for _, lang := range []string{"ru", "en"} {
+			text, _ := startText(lang, "", "")
+			if strings.Contains(text, "@") {
+				t.Errorf("lang %q: text shows a dangling @: %q", lang, text)
+			}
+		}
+		// The generic nouns are present in each language.
+		ru, _ := startText("ru", "", "")
+		if !strings.Contains(ru, "на канал") || !strings.Contains(ru, "в нашем чате") {
+			t.Errorf("russian generic fallback missing: %q", ru)
+		}
+		en, _ := startText("en", "", "")
+		if !strings.Contains(en, "Follow the channel") || !strings.Contains(en, "in our group chat") {
+			t.Errorf("english generic fallback missing: %q", en)
+		}
+	})
+	t.Run("only the missing slot degrades", func(t *testing.T) {
+		// Channel resolved, chat missing: the channel keeps its @handle, the chat is generic.
+		en, _ := startText("en", "erudit", "")
+		if !strings.Contains(en, "@erudit") || strings.Contains(en, "@erudite") {
+			t.Errorf("channel handle not shown / chat handle leaked: %q", en)
+		}
+		if !strings.Contains(en, "in our group chat") {
+			t.Errorf("chat slot did not degrade to a generic noun: %q", en)
+		}
+	})
+}