feat(variants): default a registered account to Erudit + Russian Scrabble

New Game opened on a single variant for everyone, which reads poorly on VK
where Russian Scrabble is the familiar game. A registered account now starts
with both Russian-alphabet games, so the picker offers a real choice with no
pre-selection.

A guest stays on Erudit alone and is invited to register for the rest: the
device-local guest has no profile at all, so the client-side fallback has to
keep matching the server. Registering promotes the set, but only while the
guest still carries the untouched guest default. Existing accounts are not
backfilled — the set they carry may be a deliberate choice.

The Telegram promo start-param becomes additive rather than a replacement,
with English Scrabble withheld from a Russian-speaking arrival; otherwise the
English campaign link would have taken Russian Scrabble away from every
account it onboarded.

backend/internal/inttest/promo_variant_test.go

@@ -19,10 +19,11 @@ import (
 )
 
 // TestTelegramAuthSeedsPromoVariantForNewUserOnly drives the sessions/telegram endpoint
-// to confirm a promo deep-link start-param seeds a brand-new account's variant
-// preferences (English Scrabble alongside the default Erudit), that a new account with no
-// such payload keeps the Erudit-only default, and that an existing account is never
-// re-seeded on a later login (the new-user-only contract).
+// to confirm a promo deep-link start-param widens a brand-new account's variant
+// preferences — English Scrabble on top of the default pair, but only for a
+// non-Russian-speaking arrival — that a new account with no such payload keeps the
+// default, and that an existing account is never re-seeded on a later login (the
+// new-user-only contract).
 func TestTelegramAuthSeedsPromoVariantForNewUserOnly(t *testing.T) {
 	srv := server.New(":0", server.Deps{
 		Logger:   zaptest.NewLogger(t),
@@ -33,8 +34,8 @@ func TestTelegramAuthSeedsPromoVariantForNewUserOnly(t *testing.T) {
 	})
 	h := srv.Handler()
 
-	post := func(ext, startParam string) {
-		body := `{"external_id":"` + ext + `","language_code":"en","first_name":"Promo"`
+	post := func(ext, language, startParam string) {
+		body := `{"external_id":"` + ext + `","language_code":"` + language + `","first_name":"Promo"`
 		if startParam != "" {
 			body += `,"start_param":"` + startParam + `"`
 		}
@@ -56,25 +57,33 @@ func TestTelegramAuthSeedsPromoVariantForNewUserOnly(t *testing.T) {
 		return acc.VariantPreferences
 	}
 
-	// A brand-new account reached through a promo deep-link is seeded with English
-	// Scrabble alongside the default Erudit.
+	// An English-speaking arrival on a promo deep-link gains English Scrabble on top of
+	// the default pair, so a native English player is not lost at the door.
 	promoExt := "tg-" + uuid.NewString()
-	post(promoExt, "verudit_ru-scrabble_en")
-	if got, want := reload(promoExt), []string{"erudit_ru", "scrabble_en"}; !slices.Equal(got, want) {
-		t.Errorf("promo new account variants = %v, want %v", got, want)
+	post(promoExt, "en", "verudit_ru-scrabble_en")
+	if got, want := reload(promoExt), []string{"erudit_ru", "scrabble_ru", "scrabble_en"}; !slices.Equal(got, want) {
+		t.Errorf("english promo account variants = %v, want %v", got, want)
 	}
 
-	// A brand-new account with no promo payload keeps the Erudit-only default.
+	// A Russian-speaking arrival on the same link stays on the default pair: the two
+	// Russian-alphabet games serve them, and English would only clutter New Game.
+	ruExt := "tg-" + uuid.NewString()
+	post(ruExt, "ru", "verudit_ru-scrabble_en")
+	if got, want := reload(ruExt), []string{"erudit_ru", "scrabble_ru"}; !slices.Equal(got, want) {
+		t.Errorf("russian promo account variants = %v, want %v", got, want)
+	}
+
+	// A brand-new account with no promo payload keeps the default.
 	plainExt := "tg-" + uuid.NewString()
-	post(plainExt, "")
-	if got, want := reload(plainExt), []string{"erudit_ru"}; !slices.Equal(got, want) {
+	post(plainExt, "en", "")
+	if got, want := reload(plainExt), []string{"erudit_ru", "scrabble_ru"}; !slices.Equal(got, want) {
 		t.Errorf("plain new account variants = %v, want %v", got, want)
 	}
 
 	// A later login of the promo account, even via a different payload, must not re-seed:
 	// the seed is first-contact only.
-	post(promoExt, "vscrabble_ru")
-	if got, want := reload(promoExt), []string{"erudit_ru", "scrabble_en"}; !slices.Equal(got, want) {
+	post(promoExt, "en", "vscrabble_ru")
+	if got, want := reload(promoExt), []string{"erudit_ru", "scrabble_ru", "scrabble_en"}; !slices.Equal(got, want) {
 		t.Errorf("existing account re-seeded = %v, want unchanged %v", got, want)
 	}
 }