Stage 15: dual Telegram bots & language-gated variants

Service-agnostic refinement of the owner's idea: the sign-in service returns a
set of supported game languages with the user identity, and the lobby gates the
New Game variant choice by it (en -> English; ru -> Russian + Эрудит).

- Connector hosts two bots in one container (one per service language, each its
  own token + game channel; the same telegram_id spans both). ValidateInitData
  tries each token and returns the validating bot's service_language +
  supported_languages. Per-language config (TELEGRAM_BOT_TOKEN_EN/_RU, channels).
- supported_languages rides the Session (fbs, session-scoped, not persisted); the
  UI offers only the matching variants on New Game — gating only the START of a
  new game (auto-match + friend invite), not accept/open/play; backend does not
  enforce.
- service_language persisted (accounts.service_language, migration 00010, written
  every login, last-login-wins) and routes the user-facing Notify push back
  through the right bot (push-target coalesces with preferred_language).
- Admin SendToUser/SendToGameChannel gain an operator-chosen language selector in
  the console (unrelated to ValidateInitData).
- Non-Telegram logins carry the gateway default set
  (GATEWAY_DEFAULT_SUPPORTED_LANGUAGES, all variants).

Wire (committed regen): ValidateInitDataResponse +service_language
+supported_languages; Session +supported_languages; SendToUser/SendToGameChannel
+language. Docs (ARCHITECTURE/FUNCTIONAL/_ru/READMEs) + PLAN updated; stage marked done.

backend/internal/inttest/account_test.go

@@ -155,6 +155,46 @@ func TestProvisionTelegramUnknownLanguageDefaults(t *testing.T) {
 	}
 }
 
+// TestServiceLanguageRoundTrip checks SetServiceLanguage persists the push-routing
+// language (the bot a Telegram user last signed in through): a fresh account has
+// none, a set value reads back, a later login overwrites it (last-login-wins), and
+// an empty value is a no-op. The push-target route coalesces it with the preferred
+// language.
+func TestServiceLanguageRoundTrip(t *testing.T) {
+	ctx := context.Background()
+	store := account.NewStore(testDB)
+	acc, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player")
+	if err != nil {
+		t.Fatalf("provision telegram: %v", err)
+	}
+	if acc.ServiceLanguage != "" {
+		t.Errorf("fresh ServiceLanguage = %q, want empty", acc.ServiceLanguage)
+	}
+
+	if err := store.SetServiceLanguage(ctx, acc.ID, "ru"); err != nil {
+		t.Fatalf("set service language: %v", err)
+	}
+	if got, err := store.GetByID(ctx, acc.ID); err != nil {
+		t.Fatalf("get by id: %v", err)
+	} else if got.ServiceLanguage != "ru" {
+		t.Errorf("ServiceLanguage = %q, want ru", got.ServiceLanguage)
+	}
+
+	// A later login through the other bot updates it; a subsequent empty value
+	// (a non-Telegram login) leaves it unchanged.
+	if err := store.SetServiceLanguage(ctx, acc.ID, "en"); err != nil {
+		t.Fatalf("update service language: %v", err)
+	}
+	if err := store.SetServiceLanguage(ctx, acc.ID, ""); err != nil {
+		t.Fatalf("noop service language: %v", err)
+	}
+	if got, err := store.GetByID(ctx, acc.ID); err != nil {
+		t.Fatalf("get by id: %v", err)
+	} else if got.ServiceLanguage != "en" {
+		t.Errorf("ServiceLanguage after update+noop = %q, want en", got.ServiceLanguage)
+	}
+}
+
 // TestIdentityExternalID covers the reverse identity lookup the push-target route
 // uses: it returns the external_id for the matching kind and ErrNotFound otherwise,
 // including for a guest that carries no identity.