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.

backend/internal/inttest/social_test.go

@@ -135,7 +135,7 @@ func TestFriendRequestRefusedByToggleAndBlock(t *testing.T) {
 
 	// Toggle: the addressee does not accept friend requests.
 	a, b := provisionAccount(t), provisionAccount(t)
-	if _, err := store.UpdateProfile(ctx, b, account.ProfileUpdate{DisplayName: "Player", PreferredLanguage: "en", TimeZone: "UTC", BlockFriendRequests: true}); err != nil {
+	if _, err := store.UpdateProfile(ctx, b, account.ProfileUpdate{DisplayName: "Player", PreferredLanguage: "en", TimeZone: "UTC", BlockFriendRequests: true, VariantPreferences: []string{"erudit_ru"}}); err != nil {
 		t.Fatalf("set toggle: %v", err)
 	}
 	if err := svc.SendFriendRequest(ctx, a, b); !errors.Is(err, social.ErrRequestBlocked) {
@@ -353,7 +353,7 @@ func TestChatPostListAndBlocks(t *testing.T) {
 	if _, err := svc.PostMessage(ctx, other, seats2[0], "hi", ""); err != nil {
 		t.Fatalf("post 2: %v", err)
 	}
-	if _, err := store.UpdateProfile(ctx, seats2[1], account.ProfileUpdate{DisplayName: "Player", PreferredLanguage: "en", TimeZone: "UTC", BlockChat: true}); err != nil {
+	if _, err := store.UpdateProfile(ctx, seats2[1], account.ProfileUpdate{DisplayName: "Player", PreferredLanguage: "en", TimeZone: "UTC", BlockChat: true, VariantPreferences: []string{"erudit_ru"}}); err != nil {
 		t.Fatalf("set block_chat: %v", err)
 	}
 	if msgs, _ := svc.Messages(ctx, other, seats2[1]); len(msgs) != 0 {
@@ -588,32 +588,6 @@ func TestRespondPublishesToRequester(t *testing.T) {
 	}
 }
 
-// TestNudgeRoutedByGameLanguage checks a nudge's out-of-app push carries the game's language, so
-// it is delivered by the game's bot rather than the recipient's last-login bot.
-func TestNudgeRoutedByGameLanguage(t *testing.T) {
-	ctx := context.Background()
-	svc := newSocialService()
-	pub := &capturePublisher{}
-	svc.SetNotifier(pub)
-
-	gameID, seats := newGameWithSeats(t, 2) // an English game; seat 0 is to move
-	if _, err := svc.Nudge(ctx, gameID, seats[1]); err != nil {
-		t.Fatalf("nudge: %v", err)
-	}
-	found := false
-	for _, in := range pub.intents {
-		if in.Kind == notify.KindNudge {
-			found = true
-			if in.Language != "en" {
-				t.Errorf("nudge language = %q, want en (the game's language)", in.Language)
-			}
-		}
-	}
-	if !found {
-		t.Fatal("no nudge intent published")
-	}
-}
-
 // TestAdminListMessages checks the admin moderation list: real messages only
 // (nudges excluded), the game / sender pins, the sender glob masks, and the source label.
 func TestAdminListMessages(t *testing.T) {