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/backendclient/api.go

@@ -14,26 +14,26 @@ import (
 
 // SessionResp is the credential minted by an auth operation.
 type SessionResp 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"`
 }
 
 // ProfileResp is an account's own profile.
 type ProfileResp struct {
-	UserID                 string `json:"user_id"`
-	DisplayName            string `json:"display_name"`
-	PreferredLanguage      string `json:"preferred_language"`
-	TimeZone               string `json:"time_zone"`
-	AwayStart              string `json:"away_start"`
-	AwayEnd                string `json:"away_end"`
-	HintBalance            int    `json:"hint_balance"`
-	BlockChat              bool   `json:"block_chat"`
-	BlockFriendRequests    bool   `json:"block_friend_requests"`
-	IsGuest                bool   `json:"is_guest"`
-	NotificationsInAppOnly bool   `json:"notifications_in_app_only"`
+	UserID                 string   `json:"user_id"`
+	DisplayName            string   `json:"display_name"`
+	PreferredLanguage      string   `json:"preferred_language"`
+	TimeZone               string   `json:"time_zone"`
+	AwayStart              string   `json:"away_start"`
+	AwayEnd                string   `json:"away_end"`
+	HintBalance            int      `json:"hint_balance"`
+	BlockChat              bool     `json:"block_chat"`
+	BlockFriendRequests    bool     `json:"block_friend_requests"`
+	IsGuest                bool     `json:"is_guest"`
+	NotificationsInAppOnly bool     `json:"notifications_in_app_only"`
+	VariantPreferences     []string `json:"variant_preferences"`
 	// Banner is the advertising-banner block, present only for a viewer eligible to
 	// see the banner. The gateway forwards it verbatim into the Profile payload.
 	Banner *BannerResp `json:"banner,omitempty"`
@@ -184,18 +184,15 @@ type ChatResp struct {
 }
 
 // TelegramAuth provisions/finds the Telegram account and mints a session, seeding a
-// brand-new account's display name and language from the validated launch fields and
-// recording the validating bot's serviceLanguage (which routes the account's later
-// out-of-app push).
-func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, username, firstName, serviceLanguage string) (SessionResp, error) {
+// brand-new account's display name and language from the validated launch fields.
+func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, username, firstName string) (SessionResp, error) {
 	var out SessionResp
 	err := c.do(ctx, http.MethodPost, "/api/v1/internal/sessions/telegram", "", "",
 		map[string]string{
-			"external_id":      externalID,
-			"language_code":    languageCode,
-			"username":         username,
-			"first_name":       firstName,
-			"service_language": serviceLanguage,
+			"external_id":   externalID,
+			"language_code": languageCode,
+			"username":      username,
+			"first_name":    firstName,
 		}, &out)
 	return out, err
 }