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.

gateway/internal/transcode/encode.go

@@ -12,17 +12,36 @@ import (
 // created before the table that references it, and no two tables/vectors are
 // under construction at once.
 
-// encodeSession builds a Session payload.
-func encodeSession(s backendclient.SessionResp) []byte {
+// buildSupportedLanguagesVector creates the Session.supported_languages [string]
+// vector from langs. FlatBuffers is built bottom-up, so the caller must invoke this
+// (which itself creates the element strings) before SessionStart and with no table
+// under construction.
+func buildSupportedLanguagesVector(b *flatbuffers.Builder, langs []string) flatbuffers.UOffsetT {
+	offsets := make([]flatbuffers.UOffsetT, len(langs))
+	for i, lang := range langs {
+		offsets[i] = b.CreateString(lang)
+	}
+	fb.SessionStartSupportedLanguagesVector(b, len(langs))
+	for i := len(offsets) - 1; i >= 0; i-- {
+		b.PrependUOffsetT(offsets[i])
+	}
+	return b.EndVector(len(langs))
+}
+
+// encodeSession builds a Session payload. supportedLangs is the service's set of
+// offered game languages, which the UI gates the New Game variant choice by.
+func encodeSession(s backendclient.SessionResp, supportedLangs []string) []byte {
 	b := flatbuffers.NewBuilder(128)
 	token := b.CreateString(s.Token)
 	uid := b.CreateString(s.UserID)
 	name := b.CreateString(s.DisplayName)
+	langs := buildSupportedLanguagesVector(b, supportedLangs)
 	fb.SessionStart(b)
 	fb.SessionAddToken(b, token)
 	fb.SessionAddUserId(b, uid)
 	fb.SessionAddIsGuest(b, s.IsGuest)
 	fb.SessionAddDisplayName(b, name)
+	fb.SessionAddSupportedLanguages(b, langs)
 	b.Finish(fb.SessionEnd(b))
 	return b.FinishedBytes()
 }
@@ -63,8 +82,10 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 
 // encodeLinkResult builds a LinkResult payload (Stage 11). A switched-session token
 // (a guest initiator whose durable counterpart won) is carried as a nested Session
-// for the client to adopt; it is omitted otherwise.
-func encodeLinkResult(r backendclient.LinkResultResp) []byte {
+// for the client to adopt; it is omitted otherwise. supportedLangs is the variant
+// gating set for that switched session — the link flows run on the web, so it is the
+// gateway's default (non-platform) set.
+func encodeLinkResult(r backendclient.LinkResultResp, supportedLangs []string) []byte {
 	b := flatbuffers.NewBuilder(256)
 	status := b.CreateString(r.Status)
 	secID := b.CreateString(r.SecondaryUserID)
@@ -75,11 +96,13 @@ func encodeLinkResult(r backendclient.LinkResultResp) []byte {
 		token := b.CreateString(r.Token)
 		uid := b.CreateString(r.Profile.UserID)
 		name := b.CreateString(r.Profile.DisplayName)
+		langs := buildSupportedLanguagesVector(b, supportedLangs)
 		fb.SessionStart(b)
 		fb.SessionAddToken(b, token)
 		fb.SessionAddUserId(b, uid)
 		fb.SessionAddIsGuest(b, r.Profile.IsGuest)
 		fb.SessionAddDisplayName(b, name)
+		fb.SessionAddSupportedLanguages(b, langs)
 		sess = fb.SessionEnd(b)
 	}
 	fb.LinkResultStart(b)