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/account_test.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"errors"
 	"regexp"
+	"slices"
 	"testing"
 	"time"
 
@@ -266,6 +267,81 @@ func TestProvisionSeedsTimeZone(t *testing.T) {
 	}
 }
 
+// TestVariantPreferenceDefaults covers the variant set an account is born with and how
+// registration widens it: a registered account starts on both Russian-alphabet games
+// (the column default), a guest on Эрудит alone, and promoting the guest lifts it to the
+// registered default — but only while the guest still carries that untouched set, and
+// never for an account that is already durable.
+func TestVariantPreferenceDefaults(t *testing.T) {
+	ctx := context.Background()
+	store := account.NewStore(testDB)
+
+	equal := func(t *testing.T, what string, got, want []string) {
+		t.Helper()
+		if !slices.Equal(got, want) {
+			t.Errorf("%s = %v, want %v", what, got, want)
+		}
+	}
+
+	// A registered account (any identity kind) takes the column default.
+	durable, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "ru", "", "Player", "")
+	if err != nil {
+		t.Fatalf("provision telegram: %v", err)
+	}
+	equal(t, "registered account variants", durable.VariantPreferences, account.DefaultVariantPreferences)
+
+	// A guest is deliberately narrower.
+	guest, err := store.ProvisionGuest(ctx, "", "ru")
+	if err != nil {
+		t.Fatalf("provision guest: %v", err)
+	}
+	equal(t, "guest variants", guest.VariantPreferences, account.GuestVariantPreferences)
+
+	// Registering the guest promotes it to a durable account on the registered default.
+	if err := store.ClearGuest(ctx, guest.ID); err != nil {
+		t.Fatalf("clear guest: %v", err)
+	}
+	promoted, err := store.GetByID(ctx, guest.ID)
+	if err != nil {
+		t.Fatalf("reload promoted guest: %v", err)
+	}
+	if promoted.IsGuest {
+		t.Error("promoted guest is still flagged is_guest")
+	}
+	equal(t, "promoted guest variants", promoted.VariantPreferences, account.DefaultVariantPreferences)
+
+	// A guest who picked their own set keeps it through the promotion.
+	picky, err := store.ProvisionGuest(ctx, "", "en")
+	if err != nil {
+		t.Fatalf("provision picky guest: %v", err)
+	}
+	if _, err := store.SetVariantPreferences(ctx, picky.ID, []string{"scrabble_en"}); err != nil {
+		t.Fatalf("set picky guest variants: %v", err)
+	}
+	if err := store.ClearGuest(ctx, picky.ID); err != nil {
+		t.Fatalf("clear picky guest: %v", err)
+	}
+	reloaded, err := store.GetByID(ctx, picky.ID)
+	if err != nil {
+		t.Fatalf("reload picky guest: %v", err)
+	}
+	equal(t, "picky guest variants", reloaded.VariantPreferences, []string{"scrabble_en"})
+
+	// An account that is already durable is never rewritten — an established player who
+	// settled on Эрудит alone stays there.
+	if _, err := store.SetVariantPreferences(ctx, durable.ID, []string{"erudit_ru"}); err != nil {
+		t.Fatalf("narrow durable variants: %v", err)
+	}
+	if err := store.ClearGuest(ctx, durable.ID); err != nil {
+		t.Fatalf("clear guest on a durable account: %v", err)
+	}
+	established, err := store.GetByID(ctx, durable.ID)
+	if err != nil {
+		t.Fatalf("reload durable account: %v", err)
+	}
+	equal(t, "established account variants", established.VariantPreferences, []string{"erudit_ru"})
+}
+
 // TestProvisionTelegramUnknownLanguageDefaults checks an unsupported Telegram
 // client language falls back to the account default rather than failing the
 // language CHECK.