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/server/dto.go

@@ -16,11 +16,10 @@ import (
 
 // sessionResponse is the credential returned by every auth endpoint.
 type sessionResponse struct {
-	Token           string `json:"token"`
-	UserID          string `json:"user_id"`
-	IsGuest         bool   `json:"is_guest"`
-	DisplayName     string `json:"display_name"`
-	ServiceLanguage string `json:"service_language"`
+	Token       string `json:"token"`
+	UserID      string `json:"user_id"`
+	IsGuest     bool   `json:"is_guest"`
+	DisplayName string `json:"display_name"`
 }
 
 // okResponse is a simple success acknowledgement.
@@ -49,6 +48,10 @@ type profileResponse struct {
 	BlockFriendRequests    bool   `json:"block_friend_requests"`
 	IsGuest                bool   `json:"is_guest"`
 	NotificationsInAppOnly bool   `json:"notifications_in_app_only"`
+	// VariantPreferences is the set of game variants the player allows themselves to
+	// be matched into (engine.Variant stable labels), Erudit-first. It gates the New
+	// Game picker and the matchmaker; never empty.
+	VariantPreferences []string `json:"variant_preferences"`
 	// Banner is the advertising-banner block: present only for a viewer eligible to
 	// see the banner (a free account with an empty hint wallet and without the
 	// no_banner role), absent otherwise. See banner.go.
@@ -177,11 +180,10 @@ type errorBody struct {
 // sessionResponseFor builds the credential payload for a minted session.
 func sessionResponseFor(token string, acc account.Account) sessionResponse {
 	return sessionResponse{
-		Token:           token,
-		UserID:          acc.ID.String(),
-		IsGuest:         acc.IsGuest,
-		DisplayName:     acc.DisplayName,
-		ServiceLanguage: acc.ServiceLanguage,
+		Token:       token,
+		UserID:      acc.ID.String(),
+		IsGuest:     acc.IsGuest,
+		DisplayName: acc.DisplayName,
 	}
 }
 
@@ -199,6 +201,7 @@ func profileResponseFor(acc account.Account) profileResponse {
 		BlockFriendRequests:    acc.BlockFriendRequests,
 		IsGuest:                acc.IsGuest,
 		NotificationsInAppOnly: acc.NotificationsInAppOnly,
+		VariantPreferences:     acc.VariantPreferences,
 	}
 }