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/fbs/scrabblefb/Session.go

@@ -77,8 +77,25 @@ func (rcv *Session) DisplayName() []byte {
 	return nil
 }
 
+func (rcv *Session) SupportedLanguages(j int) []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(12))
+	if o != 0 {
+		a := rcv._tab.Vector(o)
+		return rcv._tab.ByteVector(a + flatbuffers.UOffsetT(j*4))
+	}
+	return nil
+}
+
+func (rcv *Session) SupportedLanguagesLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(12))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
 func SessionStart(builder *flatbuffers.Builder) {
-	builder.StartObject(4)
+	builder.StartObject(5)
 }
 func SessionAddToken(builder *flatbuffers.Builder, token flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(token), 0)
@@ -92,6 +109,12 @@ func SessionAddIsGuest(builder *flatbuffers.Builder, isGuest bool) {
 func SessionAddDisplayName(builder *flatbuffers.Builder, displayName flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(3, flatbuffers.UOffsetT(displayName), 0)
 }
+func SessionAddSupportedLanguages(builder *flatbuffers.Builder, supportedLanguages flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(4, flatbuffers.UOffsetT(supportedLanguages), 0)
+}
+func SessionStartSupportedLanguagesVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(4, numElems, 4)
+}
 func SessionEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }