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/server/handlers_auth.go

@@ -23,10 +23,11 @@ import (
 // validated initData payload. Username, FirstName and LanguageCode seed a
 // brand-new account's display name and language; BrowserTZ (the client's detected
 // "±HH:MM" UTC offset) seeds its time zone; StartParam is the validated launch
-// deep-link payload, which may seed the new account's variant preferences (first
-// contact only). Subtype is the client-reported device family (ios/android/web);
-// Telegram's initData does not sign it, so it is recorded best-effort and the
-// payments gate never relies on it.
+// deep-link payload, which may widen the new account's variant preferences (first
+// contact only) — LanguageCode decides whether English Scrabble is part of that.
+// Subtype is the client-reported device family (ios/android/web); Telegram's initData
+// does not sign it, so it is recorded best-effort and the payments gate never relies
+// on it.
 type telegramAuthRequest struct {
 	ExternalID   string `json:"external_id"`
 	Username     string `json:"username"`
@@ -56,12 +57,14 @@ func (s *Server) handleTelegramAuth(c *gin.Context) {
 		// joined the chat before registering is granted on the spot (no chat_member
 		// event fires on registration).
 		s.publishChatAccessChange(acc.ID)
-		// A promo deep-link may seed this brand-new account's variant preferences (e.g.
-		// English Scrabble alongside the default Erudit). Best-effort: an absent or
-		// malformed payload leaves the account on its defaults, and a write failure must
-		// not block the session mint.
-		if seed := account.SeedVariantsFromStartParam(req.StartParam); len(seed) > 0 {
-			if _, err := s.accounts.SetVariantPreferences(c.Request.Context(), acc.ID, seed); err != nil {
+		// A promo deep-link may widen this brand-new account's variant preferences beyond
+		// the default (English Scrabble for a non-Russian-speaking arrival — see
+		// account.MergeVariantSeed, which gates that variant on req.LanguageCode).
+		// Best-effort: an absent or malformed payload leaves the account on its defaults,
+		// and a write failure must not block the session mint.
+		seed := account.SeedVariantsFromStartParam(req.StartParam)
+		if merged := account.MergeVariantSeed(acc.VariantPreferences, seed, req.LanguageCode); len(merged) > 0 {
+			if _, err := s.accounts.SetVariantPreferences(c.Request.Context(), acc.ID, merged); err != nil {
 				s.log.Warn("telegram: seed variant preferences failed",
 					zap.String("account", acc.ID.String()), zap.Error(err))
 			}