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.

gateway/internal/backendclient/api.go

@@ -184,10 +184,10 @@ type ChatResp struct {
 }
 
 // TelegramAuth provisions/finds the Telegram account and mints a session, seeding a
-// brand-new account's display name and language from the validated launch fields and
-// its time zone from browserTz (the client's detected "±HH:MM" UTC offset; first
-// contact only).
-func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, username, firstName, browserTz string) (SessionResp, error) {
+// brand-new account's display name and language from the validated launch fields, its
+// time zone from browserTz (the client's detected "±HH:MM" UTC offset) and, from the
+// validated launch deep-link startParam, its variant preferences (first contact only).
+func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, username, firstName, browserTz, startParam string) (SessionResp, error) {
 	var out SessionResp
 	err := c.do(ctx, http.MethodPost, "/api/v1/internal/sessions/telegram", "", "",
 		map[string]string{
@@ -196,6 +196,7 @@ func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, use
 			"username":      username,
 			"first_name":    firstName,
 			"browser_tz":    browserTz,
+			"start_param":   startParam,
 		}, &out)
 	return out, err
 }

gateway/internal/transcode/transcode.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"net/url"
 
 	"scrabble/gateway/internal/backendclient"
 	"scrabble/gateway/internal/connector"
@@ -154,11 +155,19 @@ func DomainCode(err error) (string, bool) {
 func authTelegramHandler(backend *backendclient.Client, tg TelegramValidator) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsTelegramLoginRequest(req.Payload, 0)
-		user, err := tg.ValidateInitData(ctx, string(in.InitData()))
+		initData := string(in.InitData())
+		user, err := tg.ValidateInitData(ctx, initData)
 		if err != nil {
 			return nil, err
 		}
-		sess, err := backend.TelegramAuth(ctx, user.ExternalID, user.LanguageCode, user.Username, user.FirstName, string(in.BrowserTz()))
+		// start_param rides inside the signed initData validated just above, so the launch
+		// deep-link payload can be trusted here without a separate wire field; the backend
+		// uses it to seed a brand-new account's variant preferences.
+		startParam := ""
+		if q, perr := url.ParseQuery(initData); perr == nil {
+			startParam = q.Get("start_param")
+		}
+		sess, err := backend.TelegramAuth(ctx, user.ExternalID, user.LanguageCode, user.Username, user.FirstName, string(in.BrowserTz()), startParam)
 		if err != nil {
 			return nil, err
 		}

gateway/internal/transcode/transcode_telegram_test.go

@@ -54,7 +54,7 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 		t.Fatal("auth.telegram not registered")
 	}
 
-	payload, err := op.Handler(context.Background(), transcode.Request{Payload: telegramLoginPayload("init")})
+	payload, err := op.Handler(context.Background(), transcode.Request{Payload: telegramLoginPayload("start_param=verudit_ru-scrabble_en&query_id=abc")})
 	if err != nil {
 		t.Fatalf("handler: %v", err)
 	}
@@ -66,6 +66,11 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 	if gotBody["external_id"] != "42" || gotBody["language_code"] != "ru" || gotBody["first_name"] != "Иван" {
 		t.Errorf("forwarded body = %+v, want external_id=42 language_code=ru first_name=Иван", gotBody)
 	}
+	// start_param is parsed out of the validated initData and forwarded so the backend can
+	// seed the new account's variant preferences from a promo deep link.
+	if gotBody["start_param"] != "verudit_ru-scrabble_en" {
+		t.Errorf("forwarded start_param = %q, want verudit_ru-scrabble_en", gotBody["start_param"])
+	}
 }
 
 func TestTelegramAuthInvalidInitData(t *testing.T) {