feat(telegram): promo deep-link seeds English Scrabble for new users

The promo bot button carries a configurable variant-seed start-param (default verudit_ru-scrabble_en). The gateway parses start_param from the validated initData and forwards it; the backend, on first contact only, seeds the new account variant_preferences from it (English Scrabble alongside the default Erudit).

No schema change (the scrabble_en CHECK is already in the baseline) and the gateway<->backend REST field is additive, so the rolling deploy is safe in either order. TELEGRAM_PROMO_START_PARAM configures the payload (empty forwards the user own /start payload). Covered by account unit tests, a gateway transcode test, and an integration test asserting new-only seeding.

platform/telegram/internal/promobot/promobot.go

@@ -9,6 +9,7 @@
 package promobot
 
 import (
+	"cmp"
 	"context"
 	"net/url"
 	"strings"
@@ -33,6 +34,11 @@ type Config struct {
 	// BotLinkURL is the main bot's Mini App direct link; the button appends
 	// ?startapp=<payload> to it.
 	BotLinkURL string
+	// StartParam is the campaign payload appended as ?startapp=<StartParam> to the launch
+	// button — e.g. a variant-seed deep link ("verudit_ru-scrabble_en") the backend
+	// decodes to seed a brand-new user's variant preferences. Empty falls back to
+	// forwarding the user's own /start payload.
+	StartParam string
 	// SendRatePerSecond caps outbound sends to respect the Bot API flood limits; 0
 	// disables the limiter. The burst equals the per-second rate.
 	SendRatePerSecond int
@@ -40,11 +46,12 @@ type Config struct {
 
 // Bot is the promo bot wrapper around a Telegram Bot API client.
 type Bot struct {
-	api      *tgbot.Bot
-	username string
-	linkURL  string
-	log      *zap.Logger
-	limiter  *rate.Limiter
+	api        *tgbot.Bot
+	username   string
+	linkURL    string
+	startParam string
+	log        *zap.Logger
+	limiter    *rate.Limiter
 }
 
 // New builds the promo bot, registering a /start (and default) handler that replies
@@ -53,7 +60,7 @@ func New(cfg Config, log *zap.Logger) (*Bot, error) {
 	if log == nil {
 		log = zap.NewNop()
 	}
-	t := &Bot{username: cfg.BotUsername, linkURL: cfg.BotLinkURL, log: log}
+	t := &Bot{username: cfg.BotUsername, linkURL: cfg.BotLinkURL, startParam: cfg.StartParam, log: log}
 	if cfg.SendRatePerSecond > 0 {
 		t.limiter = rate.NewLimiter(rate.Limit(cfg.SendRatePerSecond), cfg.SendRatePerSecond)
 	}
@@ -87,7 +94,8 @@ func (t *Bot) Run(ctx context.Context) {
 }
 
 // handleStart replies to any message (typically /start) with the localized promo text
-// and a button that opens the main bot's Mini App, forwarding any /start payload.
+// and a button that opens the main bot's Mini App at the configured campaign payload
+// (falling back to forwarding any /start payload the user arrived with).
 func (t *Bot) handleStart(ctx context.Context, api *tgbot.Bot, update *models.Update) {
 	if update.Message == nil {
 		return
@@ -106,9 +114,11 @@ func (t *Bot) handleStart(ctx context.Context, api *tgbot.Bot, update *models.Up
 	}
 	text, button := promoText(lang, t.username)
 	if _, err := api.SendMessage(ctx, &tgbot.SendMessageParams{
-		ChatID:      update.Message.Chat.ID,
-		Text:        text,
-		ReplyMarkup: t.launchMarkup(button, startPayload(update.Message.Text)),
+		ChatID: update.Message.Chat.ID,
+		Text:   text,
+		// The configured campaign payload (a variant-seed deep link) takes precedence;
+		// absent one, fall back to forwarding any /start payload the user arrived with.
+		ReplyMarkup: t.launchMarkup(button, cmp.Or(t.startParam, startPayload(update.Message.Text))),
 	}); err != nil {
 		t.log.Warn("promo: reply to start failed", zap.Error(err))
 	}