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/transcode_link.go

@@ -22,13 +22,15 @@ const (
 
 // registerStage11 adds the linking & merge operations. The telegram ops need the
 // connector's Login Widget validator, so they are registered only when tg is set.
-func registerStage11(r *Registry, backend *backendclient.Client, tg TelegramValidator) {
+// supportedLangs is the variant gating set for a switched link session (the link
+// flows run on the web, so the gateway default set).
+func registerStage11(r *Registry, backend *backendclient.Client, tg TelegramValidator, supportedLangs []string) {
 	r.ops[MsgLinkEmailRequest] = Op{Handler: linkEmailRequestHandler(backend), Auth: true, Email: true}
-	r.ops[MsgLinkEmailConfirm] = Op{Handler: linkEmailConfirmHandler(backend), Auth: true, Email: true}
-	r.ops[MsgLinkEmailMerge] = Op{Handler: linkEmailMergeHandler(backend), Auth: true, Email: true}
+	r.ops[MsgLinkEmailConfirm] = Op{Handler: linkEmailConfirmHandler(backend, supportedLangs), Auth: true, Email: true}
+	r.ops[MsgLinkEmailMerge] = Op{Handler: linkEmailMergeHandler(backend, supportedLangs), Auth: true, Email: true}
 	if tg != nil {
-		r.ops[MsgLinkTelegram] = Op{Handler: linkTelegramHandler(backend, tg, false), Auth: true}
-		r.ops[MsgLinkTelegramMerge] = Op{Handler: linkTelegramHandler(backend, tg, true), Auth: true}
+		r.ops[MsgLinkTelegram] = Op{Handler: linkTelegramHandler(backend, tg, false, supportedLangs), Auth: true}
+		r.ops[MsgLinkTelegramMerge] = Op{Handler: linkTelegramHandler(backend, tg, true, supportedLangs), Auth: true}
 	}
 }
 
@@ -42,31 +44,31 @@ func linkEmailRequestHandler(backend *backendclient.Client) Handler {
 	}
 }
 
-func linkEmailConfirmHandler(backend *backendclient.Client) Handler {
+func linkEmailConfirmHandler(backend *backendclient.Client, supportedLangs []string) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsLinkEmailConfirm(req.Payload, 0)
 		res, err := backend.LinkEmailConfirm(ctx, req.UserID, string(in.Email()), string(in.Code()))
 		if err != nil {
 			return nil, err
 		}
-		return encodeLinkResult(res), nil
+		return encodeLinkResult(res, supportedLangs), nil
 	}
 }
 
-func linkEmailMergeHandler(backend *backendclient.Client) Handler {
+func linkEmailMergeHandler(backend *backendclient.Client, supportedLangs []string) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsLinkEmailConfirm(req.Payload, 0)
 		res, err := backend.LinkEmailMerge(ctx, req.UserID, string(in.Email()), string(in.Code()))
 		if err != nil {
 			return nil, err
 		}
-		return encodeLinkResult(res), nil
+		return encodeLinkResult(res, supportedLangs), nil
 	}
 }
 
 // linkTelegramHandler validates Login Widget data via the connector and then calls
 // the backend's link or merge endpoint with the trusted Telegram external id.
-func linkTelegramHandler(backend *backendclient.Client, tg TelegramValidator, merge bool) Handler {
+func linkTelegramHandler(backend *backendclient.Client, tg TelegramValidator, merge bool, supportedLangs []string) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsLinkTelegramRequest(req.Payload, 0)
 		user, err := tg.ValidateLoginWidget(ctx, string(in.Data()))
@@ -82,6 +84,6 @@ func linkTelegramHandler(backend *backendclient.Client, tg TelegramValidator, me
 		if err != nil {
 			return nil, err
 		}
-		return encodeLinkResult(res), nil
+		return encodeLinkResult(res, supportedLangs), nil
 	}
 }