feat(telegram,game): single bot + per-user variant preferences

Collapse the two per-language Telegram bots into one unified bot and
replace language-based variant gating with explicit per-user variant
preferences.

- Telegram: one bot; drop service_language and the supported_languages
  set everywhere (DB, account, auth, FlatBuffers Session wire, gateway,
  connector proto). The single bot renders chat and out-of-app push in
  the recipient's preferred_language; remove the game-language push
  routing override (notify Intent.Language / push Event.language).
- Preferences: new accounts.variant_preferences (text[], DB default
  {erudit_ru}, CHECK non-empty + subset of the three variants). Gates
  the New Game picker, vs-AI and the friend invitation the player
  creates, enforced server-side (HTTP 400 otherwise); an invited friend
  may still accept any variant. Edited on the Settings screen; variants
  are Erudit-first everywhere.
- Admin: drop the per-bot language selectors (broadcast / send-to-user)
  and the feedback channel_lang column/field.
- Env/CI: collapse TELEGRAM_BOT_TOKEN_{EN,RU}, TELEGRAM_GAME_CHANNEL_ID_{EN,RU},
  VITE_TELEGRAM_LINK{,_EN,_RU} and VITE_TELEGRAM_GAME_CHANNEL_NAME_{EN,RU}
  to single unsuffixed names; drop GATEWAY_DEFAULT_SUPPORTED_LANGUAGES.
- Docs updated (ARCHITECTURE, FUNCTIONAL + _ru, platform/telegram, gateway,
  backend, ui, UI_DESIGN, PRERELEASE).

The migration squash is deferred to a follow-up PR.

gateway/internal/config/config.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"os"
 	"strconv"
-	"strings"
 	"time"
 
 	pkgtel "scrabble/pkg/telemetry"
@@ -33,11 +32,6 @@ 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
@@ -85,13 +79,6 @@ const (
 // yet small enough to stop a cheap memory-amplification upload.
 const DefaultMaxBodyBytes = 1 << 20
 
-// 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{
@@ -141,9 +128,6 @@ func Load() (Config, error) {
 	if c.MaxBodyBytes, err = envInt("GATEWAY_MAX_BODY_BYTES", DefaultMaxBodyBytes); 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
 	}
@@ -190,31 +174,6 @@ 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) {