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_grpc.pb.go

@@ -58,12 +58,13 @@ type TelegramClient interface {
 	// localized message with a Mini App deep-link button from the FlatBuffers
 	// payload; unrenderable kinds are skipped (delivered=false).
 	Notify(ctx context.Context, in *NotifyRequest, opts ...grpc.CallOption) (*NotifyResponse, error)
-	// 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.
 	SendToUser(ctx context.Context, in *SendToUserRequest, opts ...grpc.CallOption) (*SendResponse, error)
-	// 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.
 	SendToGameChannel(ctx context.Context, in *SendToGameChannelRequest, opts ...grpc.CallOption) (*SendResponse, error)
 }
 
@@ -146,12 +147,13 @@ type TelegramServer interface {
 	// localized message with a Mini App deep-link button from the FlatBuffers
 	// payload; unrenderable kinds are skipped (delivered=false).
 	Notify(context.Context, *NotifyRequest) (*NotifyResponse, error)
-	// 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.
 	SendToUser(context.Context, *SendToUserRequest) (*SendResponse, error)
-	// 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.
 	SendToGameChannel(context.Context, *SendToGameChannelRequest) (*SendResponse, error)
 	mustEmbedUnimplementedTelegramServer()
 }