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.

pkg/proto/telegram/v1/telegram_grpc.pb.go

@@ -58,13 +58,11 @@ type TelegramClient interface {
 	// localized message with a Mini App deep-link button from the FlatBuffers
 	// payload; unrenderable kinds are skipped (delivered=false).
 	Notify(ctx context.Context, in *NotifyRequest, opts ...grpc.CallOption) (*NotifyResponse, error)
-	// SendToUser sends an arbitrary text message to one user through the bot the
-	// request selects by language (admin use). delivered is false
-	// when the user has not started that bot.
+	// SendToUser sends an arbitrary text message to one user (admin use). delivered is
+	// false when the user has not started the bot.
 	SendToUser(ctx context.Context, in *SendToUserRequest, opts ...grpc.CallOption) (*SendResponse, error)
-	// SendToGameChannel posts an arbitrary text message to the game channel of the
-	// bot the request selects by language (admin use); the channel
-	// ids live only in the connector configuration.
+	// SendToGameChannel posts an arbitrary text message to the bot's game channel
+	// (admin use); the channel id lives only in the connector configuration.
 	SendToGameChannel(ctx context.Context, in *SendToGameChannelRequest, opts ...grpc.CallOption) (*SendResponse, error)
 }
 
@@ -147,13 +145,11 @@ type TelegramServer interface {
 	// localized message with a Mini App deep-link button from the FlatBuffers
 	// payload; unrenderable kinds are skipped (delivered=false).
 	Notify(context.Context, *NotifyRequest) (*NotifyResponse, error)
-	// SendToUser sends an arbitrary text message to one user through the bot the
-	// request selects by language (admin use). delivered is false
-	// when the user has not started that bot.
+	// SendToUser sends an arbitrary text message to one user (admin use). delivered is
+	// false when the user has not started the bot.
 	SendToUser(context.Context, *SendToUserRequest) (*SendResponse, error)
-	// SendToGameChannel posts an arbitrary text message to the game channel of the
-	// bot the request selects by language (admin use); the channel
-	// ids live only in the connector configuration.
+	// SendToGameChannel posts an arbitrary text message to the bot's game channel
+	// (admin use); the channel id lives only in the connector configuration.
 	SendToGameChannel(context.Context, *SendToGameChannelRequest) (*SendResponse, error)
 	mustEmbedUnimplementedTelegramServer()
 }