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.

ui/src/gen/fbs/scrabblefb/session.ts

@@ -46,8 +46,20 @@ displayName(optionalEncoding?:any):string|Uint8Array|null {
   return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
 }
 
+supportedLanguages(index: number):string
+supportedLanguages(index: number,optionalEncoding:flatbuffers.Encoding):string|Uint8Array
+supportedLanguages(index: number,optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 12);
+  return offset ? this.bb!.__string(this.bb!.__vector(this.bb_pos + offset) + index * 4, optionalEncoding) : null;
+}
+
+supportedLanguagesLength():number {
+  const offset = this.bb!.__offset(this.bb_pos, 12);
+  return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
+}
+
 static startSession(builder:flatbuffers.Builder) {
-  builder.startObject(4);
+  builder.startObject(5);
 }
 
 static addToken(builder:flatbuffers.Builder, tokenOffset:flatbuffers.Offset) {
@@ -66,17 +78,34 @@ static addDisplayName(builder:flatbuffers.Builder, displayNameOffset:flatbuffers
   builder.addFieldOffset(3, displayNameOffset, 0);
 }
 
+static addSupportedLanguages(builder:flatbuffers.Builder, supportedLanguagesOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(4, supportedLanguagesOffset, 0);
+}
+
+static createSupportedLanguagesVector(builder:flatbuffers.Builder, data:flatbuffers.Offset[]):flatbuffers.Offset {
+  builder.startVector(4, data.length, 4);
+  for (let i = data.length - 1; i >= 0; i--) {
+    builder.addOffset(data[i]!);
+  }
+  return builder.endVector();
+}
+
+static startSupportedLanguagesVector(builder:flatbuffers.Builder, numElems:number) {
+  builder.startVector(4, numElems, 4);
+}
+
 static endSession(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createSession(builder:flatbuffers.Builder, tokenOffset:flatbuffers.Offset, userIdOffset:flatbuffers.Offset, isGuest:boolean, displayNameOffset:flatbuffers.Offset):flatbuffers.Offset {
+static createSession(builder:flatbuffers.Builder, tokenOffset:flatbuffers.Offset, userIdOffset:flatbuffers.Offset, isGuest:boolean, displayNameOffset:flatbuffers.Offset, supportedLanguagesOffset:flatbuffers.Offset):flatbuffers.Offset {
   Session.startSession(builder);
   Session.addToken(builder, tokenOffset);
   Session.addUserId(builder, userIdOffset);
   Session.addIsGuest(builder, isGuest);
   Session.addDisplayName(builder, displayNameOffset);
+  Session.addSupportedLanguages(builder, supportedLanguagesOffset);
   return Session.endSession(builder);
 }
 }