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.

gateway/internal/config/config.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"os"
 	"strconv"
+	"strings"
 	"time"
 
 	pkgtel "scrabble/pkg/telemetry"
@@ -32,6 +33,11 @@ type Config struct {
 	// gateway calls it to validate Mini App initData and to deliver out-of-app push.
 	// Empty disables the telegram auth path and the out-of-app push channel.
 	ConnectorAddr string
+	// DefaultSupportedLanguages is the New Game variant gating set put on the Session
+	// for non-platform logins (web / email / guest), which carry no service container
+	// to declare one. The UI offers only variants in this set (en -> English; ru ->
+	// Russian + Эрудит). Defaults to all of them; a deployment may narrow it.
+	DefaultSupportedLanguages []string
 	// SessionTTL bounds how long a resolved session stays cached; SessionCacheMax
 	// caps the number of cached sessions.
 	SessionTTL      time.Duration
@@ -71,6 +77,13 @@ const (
 	defaultServiceName           = "scrabble-gateway"
 )
 
+// supportedLanguages is the set of game languages a service may declare for the
+// New Game variant gating; defaultSupportedLanguages is the non-platform fallback.
+var (
+	supportedLanguages        = map[string]bool{"en": true, "ru": true}
+	defaultSupportedLanguages = []string{"en", "ru"}
+)
+
 // DefaultRateLimit returns the built-in anti-abuse limits.
 func DefaultRateLimit() RateLimitConfig {
 	return RateLimitConfig{
@@ -113,6 +126,9 @@ func Load() (Config, error) {
 	if c.PushHeartbeatInterval, err = envDuration("GATEWAY_PUSH_HEARTBEAT_INTERVAL", defaultPushHeartbeatInterval); err != nil {
 		return Config{}, err
 	}
+	if c.DefaultSupportedLanguages, err = envLanguages("GATEWAY_DEFAULT_SUPPORTED_LANGUAGES", defaultSupportedLanguages); err != nil {
+		return Config{}, err
+	}
 	if err := c.validate(); err != nil {
 		return Config{}, err
 	}
@@ -156,6 +172,31 @@ func envOr(key, fallback string) string {
 	return fallback
 }
 
+// envLanguages parses a comma-separated language list (e.g. "en,ru") from the
+// environment variable named key, returning fallback when it is unset. Every entry
+// must be a supported language and the result must be non-empty.
+func envLanguages(key string, fallback []string) ([]string, error) {
+	raw := strings.TrimSpace(os.Getenv(key))
+	if raw == "" {
+		return fallback, nil
+	}
+	var out []string
+	for part := range strings.SplitSeq(raw, ",") {
+		lang := strings.ToLower(strings.TrimSpace(part))
+		if lang == "" {
+			continue
+		}
+		if !supportedLanguages[lang] {
+			return nil, fmt.Errorf("config: %s: unsupported language %q", key, lang)
+		}
+		out = append(out, lang)
+	}
+	if len(out) == 0 {
+		return nil, fmt.Errorf("config: %s must list at least one language", key)
+	}
+	return out, nil
+}
+
 // envInt parses the environment variable named key as an int, returning fallback
 // when it is unset and an error when it is set but malformed.
 func envInt(key string, fallback int) (int, error) {