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/email_test.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"errors"
 	"regexp"
+	"slices"
 	"testing"
 	"time"
 
@@ -134,12 +135,20 @@ func TestUpdateProfilePersists(t *testing.T) {
 	store := account.NewStore(testDB)
 	acc := provisionAccount(t)
 
+	// A fresh account defaults to Erudit only (the DB-level column default).
+	if def, err := store.GetByID(ctx, acc); err != nil {
+		t.Fatalf("get default: %v", err)
+	} else if want := []string{"erudit_ru"}; !slices.Equal(def.VariantPreferences, want) {
+		t.Errorf("default variant preferences = %v, want %v", def.VariantPreferences, want)
+	}
+
 	updated, err := store.UpdateProfile(ctx, acc, account.ProfileUpdate{
 		DisplayName:         "Kaya",
 		PreferredLanguage:   "ru",
 		TimeZone:            "Europe/Moscow",
 		BlockChat:           true,
 		BlockFriendRequests: true,
+		VariantPreferences:  []string{"scrabble_en", "erudit_ru"},
 	})
 	if err != nil {
 		t.Fatalf("update profile: %v", err)
@@ -157,6 +166,10 @@ func TestUpdateProfilePersists(t *testing.T) {
 	if reloaded.TimeZone != "Europe/Moscow" || !reloaded.BlockChat {
 		t.Errorf("profile did not persist: %+v", reloaded)
 	}
+	// The text[] column round-trips and is stored canonically (Erudit-first).
+	if want := []string{"erudit_ru", "scrabble_en"}; !slices.Equal(reloaded.VariantPreferences, want) {
+		t.Errorf("variant preferences = %v, want %v", reloaded.VariantPreferences, want)
+	}
 }
 
 // TestUpdateProfileOffsetTimezone checks the UTC-offset timezone: it is
@@ -167,9 +180,10 @@ func TestUpdateProfileOffsetTimezone(t *testing.T) {
 	acc := provisionAccount(t)
 
 	updated, err := store.UpdateProfile(ctx, acc, account.ProfileUpdate{
-		DisplayName:       "Kaya",
-		PreferredLanguage: "en",
-		TimeZone:          "+03:00",
+		DisplayName:        "Kaya",
+		PreferredLanguage:  "en",
+		TimeZone:           "+03:00",
+		VariantPreferences: []string{"erudit_ru"},
 	})
 	if err != nil {
 		t.Fatalf("update with offset timezone: %v", err)