feat(telegram,game): single bot + per-user variant preferences

Collapse the two per-language Telegram bots into one unified bot and
replace language-based variant gating with explicit per-user variant
preferences.

- Telegram: one bot; drop service_language and the supported_languages
  set everywhere (DB, account, auth, FlatBuffers Session wire, gateway,
  connector proto). The single bot renders chat and out-of-app push in
  the recipient's preferred_language; remove the game-language push
  routing override (notify Intent.Language / push Event.language).
- Preferences: new accounts.variant_preferences (text[], DB default
  {erudit_ru}, CHECK non-empty + subset of the three variants). Gates
  the New Game picker, vs-AI and the friend invitation the player
  creates, enforced server-side (HTTP 400 otherwise); an invited friend
  may still accept any variant. Edited on the Settings screen; variants
  are Erudit-first everywhere.
- Admin: drop the per-bot language selectors (broadcast / send-to-user)
  and the feedback channel_lang column/field.
- Env/CI: collapse TELEGRAM_BOT_TOKEN_{EN,RU}, TELEGRAM_GAME_CHANNEL_ID_{EN,RU},
  VITE_TELEGRAM_LINK{,_EN,_RU} and VITE_TELEGRAM_GAME_CHANNEL_NAME_{EN,RU}
  to single unsuffixed names; drop GATEWAY_DEFAULT_SUPPORTED_LANGUAGES.
- Docs updated (ARCHITECTURE, FUNCTIONAL + _ru, platform/telegram, gateway,
  backend, ui, UI_DESIGN, PRERELEASE).

The migration squash is deferred to a follow-up PR.

gateway/internal/transcode/encode.go

@@ -13,38 +13,17 @@ import (
 // created before the table that references it, and no two tables/vectors are
 // under construction at once.
 
-// 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 {
+// encodeSession builds a Session payload.
+func encodeSession(s backendclient.SessionResp) []byte {
 	b := flatbuffers.NewBuilder(128)
 	token := b.CreateString(s.Token)
 	uid := b.CreateString(s.UserID)
 	name := b.CreateString(s.DisplayName)
-	svcLang := b.CreateString(s.ServiceLanguage)
-	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)
-	fb.SessionAddServiceLanguage(b, svcLang)
 	b.Finish(fb.SessionEnd(b))
 	return b.FinishedBytes()
 }
@@ -74,6 +53,7 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	if p.Banner != nil {
 		banner = encodeBanner(b, *p.Banner)
 	}
+	prefs := buildStringVector(b, p.VariantPreferences, fb.ProfileStartVariantPreferencesVector)
 	fb.ProfileStart(b)
 	fb.ProfileAddUserId(b, uid)
 	fb.ProfileAddDisplayName(b, name)
@@ -86,6 +66,7 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	fb.ProfileAddAwayStart(b, awayStart)
 	fb.ProfileAddAwayEnd(b, awayEnd)
 	fb.ProfileAddNotificationsInAppOnly(b, p.NotificationsInAppOnly)
+	fb.ProfileAddVariantPreferences(b, prefs)
 	if p.Banner != nil {
 		fb.ProfileAddBanner(b, banner)
 	}
@@ -147,10 +128,8 @@ func encodeBlockStatus(s backendclient.BlockStatusResp) []byte {
 
 // encodeLinkResult builds a LinkResult payload. 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. 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 {
+// for the client to adopt; it is omitted otherwise.
+func encodeLinkResult(r backendclient.LinkResultResp) []byte {
 	b := flatbuffers.NewBuilder(256)
 	status := b.CreateString(r.Status)
 	secID := b.CreateString(r.SecondaryUserID)
@@ -161,13 +140,11 @@ func encodeLinkResult(r backendclient.LinkResultResp, supportedLangs []string) [
 		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)