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/feedback_test.go

@@ -145,48 +145,33 @@ func TestFeedbackReplyHiddenAfterNewMessage(t *testing.T) {
 	}
 }
 
-func TestFeedbackSnapshotsLanguages(t *testing.T) {
+func TestFeedbackSnapshotsLanguage(t *testing.T) {
 	ctx := context.Background()
 	svc := newFeedbackService()
 	acc := provisionAccount(t)
 	if _, err := testDB.ExecContext(ctx,
-		`UPDATE backend.accounts SET preferred_language = 'en', service_language = 'ru' WHERE account_id = $1`, acc); err != nil {
-		t.Fatalf("set languages: %v", err)
+		`UPDATE backend.accounts SET preferred_language = 'en' WHERE account_id = $1`, acc); err != nil {
+		t.Fatalf("set language: %v", err)
 	}
-	// A Telegram (connector) message snapshots both the interface language and the bot.
+	// A message snapshots the sender's interface language at submit time.
 	if err := svc.Submit(ctx, acc, "from telegram", nil, "", "telegram", ""); err != nil {
 		t.Fatalf("submit: %v", err)
 	}
 	id := latestFeedbackID(t, svc, acc)
 	if m, err := svc.AdminGet(ctx, id); err != nil {
 		t.Fatalf("admin get: %v", err)
-	} else if m.Lang != "en" || m.ChannelLang != "ru" {
-		t.Fatalf("snapshot = lang %q / channel_lang %q, want en / ru", m.Lang, m.ChannelLang)
+	} else if m.Lang != "en" {
+		t.Fatalf("snapshot = lang %q, want en", m.Lang)
 	}
 	// Changing the account afterwards must not change the stored snapshot.
 	if _, err := testDB.ExecContext(ctx,
-		`UPDATE backend.accounts SET preferred_language = 'ru', service_language = 'en' WHERE account_id = $1`, acc); err != nil {
-		t.Fatalf("change languages: %v", err)
+		`UPDATE backend.accounts SET preferred_language = 'ru' WHERE account_id = $1`, acc); err != nil {
+		t.Fatalf("change language: %v", err)
 	}
 	if m, err := svc.AdminGet(ctx, id); err != nil {
 		t.Fatal(err)
-	} else if m.Lang != "en" || m.ChannelLang != "ru" {
-		t.Fatalf("snapshot drifted after account change = lang %q / channel_lang %q", m.Lang, m.ChannelLang)
-	}
-
-	// A non-connector channel records no bot language even when the account has one.
-	acc2 := provisionAccount(t)
-	if _, err := testDB.ExecContext(ctx,
-		`UPDATE backend.accounts SET preferred_language = 'en', service_language = 'ru' WHERE account_id = $1`, acc2); err != nil {
-		t.Fatalf("set languages 2: %v", err)
-	}
-	if err := svc.Submit(ctx, acc2, "from web", nil, "", "web", ""); err != nil {
-		t.Fatalf("submit web: %v", err)
-	}
-	if m, err := svc.AdminGet(ctx, latestFeedbackID(t, svc, acc2)); err != nil {
-		t.Fatal(err)
-	} else if m.Lang != "en" || m.ChannelLang != "" {
-		t.Fatalf("web snapshot = lang %q / channel_lang %q, want en / empty", m.Lang, m.ChannelLang)
+	} else if m.Lang != "en" {
+		t.Fatalf("snapshot drifted after account change = lang %q, want en", m.Lang)
 	}
 }