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.

backend/internal/connector/client.go

@@ -1,8 +1,9 @@
 // Package connector is the backend's gRPC client for the Telegram platform
 // connector side-service. The admin console uses it to send operator broadcasts:
-// a direct message to one user, or a post to the connector's configured game
-// channel. The connector lives on the trusted internal network, so the
-// connection uses insecure (plaintext) transport credentials
+// a direct message to one user, or a post to a game channel. Each broadcast
+// selects the delivering bot by language (an operator choice, since the connector
+// hosts one bot per service language). The connector lives on the trusted internal
+// network, so the connection uses insecure (plaintext) transport credentials
 // (docs/ARCHITECTURE.md §12). It mirrors gateway/internal/connector, narrowed to
 // the two broadcast methods the admin surface needs.
 package connector
@@ -36,21 +37,22 @@ func New(addr string) (*Client, error) {
 func (c *Client) Close() error { return c.conn.Close() }
 
 // SendToUser sends an operator text message to one user, addressed by their
-// platform external_id. delivered reports whether the connector actually sent it
-// (false when the user has not started the bot).
-func (c *Client) SendToUser(ctx context.Context, externalID, text string) (bool, error) {
-	resp, err := c.c.SendToUser(ctx, &telegramv1.SendToUserRequest{ExternalId: externalID, Text: text})
+// platform external_id, through the bot for the given language. delivered reports
+// whether the connector actually sent it (false when the user has not started that
+// bot).
+func (c *Client) SendToUser(ctx context.Context, externalID, text, language string) (bool, error) {
+	resp, err := c.c.SendToUser(ctx, &telegramv1.SendToUserRequest{ExternalId: externalID, Text: text, Language: language})
 	if err != nil {
 		return false, err
 	}
 	return resp.GetDelivered(), nil
 }
 
-// SendToGameChannel posts an operator text message to the connector's configured
-// game channel. delivered reports whether the connector sent it (false when no
-// channel is configured).
-func (c *Client) SendToGameChannel(ctx context.Context, text string) (bool, error) {
-	resp, err := c.c.SendToGameChannel(ctx, &telegramv1.SendToGameChannelRequest{Text: text})
+// SendToGameChannel posts an operator text message to the game channel of the bot
+// for the given language. delivered reports whether the connector sent it (false
+// when that bot has no channel configured).
+func (c *Client) SendToGameChannel(ctx context.Context, text, language string) (bool, error) {
+	resp, err := c.c.SendToGameChannel(ctx, &telegramv1.SendToGameChannelRequest{Text: text, Language: language})
 	if err != nil {
 		return false, err
 	}