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.proto

@@ -31,13 +31,11 @@ service Telegram {
   // localized message with a Mini App deep-link button from the FlatBuffers
   // payload; unrenderable kinds are skipped (delivered=false).
   rpc Notify(NotifyRequest) returns (NotifyResponse);
-  // 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.
   rpc SendToUser(SendToUserRequest) returns (SendResponse);
-  // 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.
   rpc SendToGameChannel(SendToGameChannelRequest) returns (SendResponse);
 }
 
@@ -48,19 +46,12 @@ message ValidateInitDataRequest {
 
 // ValidateInitDataResponse is the validated identity. external_id is the Telegram
 // user id used as the identities external_id; language_code seeds a new account's
-// preferred (interface) language. service_language (en/ru) is the language tag of
-// the bot that validated the launch data; it is persisted per account and routes
-// the user's out-of-app push back through the right bot (it is NOT the game's
-// language). supported_languages is that bot's set of offered game languages
-// (subset of {en, ru}, at least one — a singleton for a single-language bot); the
-// UI gates the New Game variant choice by it.
+// preferred (interface) language.
 message ValidateInitDataResponse {
   string external_id = 1;
   string username = 2;
   string first_name = 3;
   string language_code = 4;
-  string service_language = 5;
-  repeated string supported_languages = 6;
 }
 
 // ValidateLoginWidgetRequest carries the Login Widget result serialized as a URL
@@ -80,9 +71,8 @@ message ValidateLoginWidgetResponse {
 
 // NotifyRequest addresses a push event to one recipient. kind is the backend push
 // catalog kind (your_turn, nudge, match_found, notify); payload is the FlatBuffers
-// scrabblefb.* body for that kind; language (en/ru) is the recipient's service
-// language (from their last ValidateInitData) — it both selects the delivering bot
-// and the message template.
+// scrabblefb.* body for that kind; language (en/ru) is the recipient's interface
+// language, used to render the message template (the single bot needs no routing).
 message NotifyRequest {
   string external_id = 1;
   string kind = 2;
@@ -96,21 +86,15 @@ message NotifyResponse {
   bool delivered = 1;
 }
 
-// SendToUserRequest is an admin text message to one user by external_id. language
-// (en/ru) selects which bot delivers it — an operator choice in the admin console,
-// unrelated to the user's service language.
+// SendToUserRequest is an admin text message to one user by external_id.
 message SendToUserRequest {
   string external_id = 1;
   string text = 2;
-  string language = 3;
 }
 
-// SendToGameChannelRequest is an admin text message to a game channel. language
-// (en/ru) selects which bot's configured channel receives it — an operator choice
-// in the admin console.
+// SendToGameChannelRequest is an admin text message to the bot's game channel.
 message SendToGameChannelRequest {
   string text = 1;
-  string language = 2;
 }
 
 // SendResponse reports whether the message was sent.