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/postgres/jet/backend/model/accounts.go

@@ -29,4 +29,5 @@ type Accounts struct {
 	PaidAccount            bool
 	MergedInto             *uuid.UUID
 	MergedAt               *time.Time
+	ServiceLanguage        *string
 }

backend/internal/postgres/jet/backend/table/accounts.go

@@ -33,6 +33,7 @@ type accountsTable struct {
 	PaidAccount            postgres.ColumnBool
 	MergedInto             postgres.ColumnString
 	MergedAt               postgres.ColumnTimestampz
+	ServiceLanguage        postgres.ColumnString
 
 	AllColumns     postgres.ColumnList
 	MutableColumns postgres.ColumnList
@@ -90,8 +91,9 @@ func newAccountsTableImpl(schemaName, tableName, alias string) accountsTable {
 		PaidAccountColumn            = postgres.BoolColumn("paid_account")
 		MergedIntoColumn             = postgres.StringColumn("merged_into")
 		MergedAtColumn               = postgres.TimestampzColumn("merged_at")
-		allColumns                   = postgres.ColumnList{AccountIDColumn, DisplayNameColumn, PreferredLanguageColumn, TimeZoneColumn, BlockChatColumn, BlockFriendRequestsColumn, CreatedAtColumn, UpdatedAtColumn, AwayStartColumn, AwayEndColumn, HintBalanceColumn, IsGuestColumn, NotificationsInAppOnlyColumn, PaidAccountColumn, MergedIntoColumn, MergedAtColumn}
-		mutableColumns               = postgres.ColumnList{DisplayNameColumn, PreferredLanguageColumn, TimeZoneColumn, BlockChatColumn, BlockFriendRequestsColumn, CreatedAtColumn, UpdatedAtColumn, AwayStartColumn, AwayEndColumn, HintBalanceColumn, IsGuestColumn, NotificationsInAppOnlyColumn, PaidAccountColumn, MergedIntoColumn, MergedAtColumn}
+		ServiceLanguageColumn        = postgres.StringColumn("service_language")
+		allColumns                   = postgres.ColumnList{AccountIDColumn, DisplayNameColumn, PreferredLanguageColumn, TimeZoneColumn, BlockChatColumn, BlockFriendRequestsColumn, CreatedAtColumn, UpdatedAtColumn, AwayStartColumn, AwayEndColumn, HintBalanceColumn, IsGuestColumn, NotificationsInAppOnlyColumn, PaidAccountColumn, MergedIntoColumn, MergedAtColumn, ServiceLanguageColumn}
+		mutableColumns               = postgres.ColumnList{DisplayNameColumn, PreferredLanguageColumn, TimeZoneColumn, BlockChatColumn, BlockFriendRequestsColumn, CreatedAtColumn, UpdatedAtColumn, AwayStartColumn, AwayEndColumn, HintBalanceColumn, IsGuestColumn, NotificationsInAppOnlyColumn, PaidAccountColumn, MergedIntoColumn, MergedAtColumn, ServiceLanguageColumn}
 		defaultColumns               = postgres.ColumnList{DisplayNameColumn, PreferredLanguageColumn, TimeZoneColumn, BlockChatColumn, BlockFriendRequestsColumn, CreatedAtColumn, UpdatedAtColumn, AwayStartColumn, AwayEndColumn, HintBalanceColumn, IsGuestColumn, NotificationsInAppOnlyColumn, PaidAccountColumn}
 	)
 
@@ -115,6 +117,7 @@ func newAccountsTableImpl(schemaName, tableName, alias string) accountsTable {
 		PaidAccount:            PaidAccountColumn,
 		MergedInto:             MergedIntoColumn,
 		MergedAt:               MergedAtColumn,
+		ServiceLanguage:        ServiceLanguageColumn,
 
 		AllColumns:     allColumns,
 		MutableColumns: mutableColumns,

backend/internal/postgres/migrations/00010_service_language.sql

@@ -0,0 +1,21 @@
+-- +goose Up
+-- Stage 15 dual Telegram bots: service_language records the language tag of the bot
+-- a Telegram user last authenticated through (their last ValidateInitData). It is
+-- updated on every Telegram login — new and existing accounts — and routes the
+-- user's out-of-app push back through the right bot. It is distinct from
+-- preferred_language (the interface language) and from a game's variant language.
+-- Nullable: an account that has never signed in through a tagged bot (legacy,
+-- email-only or guest) has no value, and push routing falls back to
+-- preferred_language. Adds a column, so the generated jet code is regenerated
+-- (cmd/jetgen).
+SET search_path = backend, pg_catalog;
+
+ALTER TABLE accounts
+    ADD COLUMN service_language text
+        CHECK (service_language IN ('en', 'ru'));
+
+-- +goose Down
+SET search_path = backend, pg_catalog;
+
+ALTER TABLE accounts
+    DROP COLUMN service_language;