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/transcode_link.go

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