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.

pkg/proto/telegram/v1/telegram.proto

@@ -31,12 +31,13 @@ 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 (admin use, wired in
-  // Stage 10). delivered is false when the user has not started the bot.
+  // SendToUser sends an arbitrary text message to one user through the bot the
+  // request selects by language (admin use, wired in Stage 10). delivered is false
+  // when the user has not started that bot.
   rpc SendToUser(SendToUserRequest) returns (SendResponse);
-  // SendToGameChannel posts an arbitrary text message to the bot's configured
-  // game channel (admin use, wired in Stage 10); the channel id lives only in the
-  // connector configuration.
+  // SendToGameChannel posts an arbitrary text message to the game channel of the
+  // bot the request selects by language (admin use, wired in Stage 10); the channel
+  // ids live only in the connector configuration.
   rpc SendToGameChannel(SendToGameChannelRequest) returns (SendResponse);
 }
 
@@ -47,12 +48,19 @@ 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 language.
+// 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.
 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
@@ -72,7 +80,9 @@ 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) selects the message template.
+// 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.
 message NotifyRequest {
   string external_id = 1;
   string kind = 2;
@@ -86,15 +96,21 @@ message NotifyResponse {
   bool delivered = 1;
 }
 
-// SendToUserRequest is an admin text message to one user by external_id.
+// 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.
 message SendToUserRequest {
   string external_id = 1;
   string text = 2;
+  string language = 3;
 }
 
-// SendToGameChannelRequest is an admin text message to the configured game channel.
+// 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.
 message SendToGameChannelRequest {
   string text = 1;
+  string language = 2;
 }
 
 // SendResponse reports whether the message was sent.