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/connector/client.go

@@ -1,11 +1,10 @@
 // Package connector is the backend's gRPC client for the Telegram platform
 // connector side-service. The admin console uses it to send operator broadcasts:
-// a direct message to one user, or a post to a game channel. Each broadcast
-// selects the delivering bot by language (an operator choice, since the connector
-// hosts one bot per service language). The connector lives on the trusted internal
-// network, so the connection uses insecure (plaintext) transport credentials
-// (docs/ARCHITECTURE.md §12). It mirrors gateway/internal/connector, narrowed to
-// the two broadcast methods the admin surface needs.
+// a direct message to one user, or a post to the game channel, through the single
+// bot. The connector lives on the trusted internal network, so the connection uses
+// insecure (plaintext) transport credentials (docs/ARCHITECTURE.md §12). It mirrors
+// gateway/internal/connector, narrowed to the two broadcast methods the admin
+// surface needs.
 package connector
 
 import (
@@ -37,22 +36,21 @@ func New(addr string) (*Client, error) {
 func (c *Client) Close() error { return c.conn.Close() }
 
 // SendToUser sends an operator text message to one user, addressed by their
-// platform external_id, through the bot for the given language. delivered reports
-// whether the connector actually sent it (false when the user has not started that
-// bot).
-func (c *Client) SendToUser(ctx context.Context, externalID, text, language string) (bool, error) {
-	resp, err := c.c.SendToUser(ctx, &telegramv1.SendToUserRequest{ExternalId: externalID, Text: text, Language: language})
+// platform external_id, through the bot. delivered reports whether the connector
+// actually sent it (false when the user has not started the bot).
+func (c *Client) SendToUser(ctx context.Context, externalID, text string) (bool, error) {
+	resp, err := c.c.SendToUser(ctx, &telegramv1.SendToUserRequest{ExternalId: externalID, Text: text})
 	if err != nil {
 		return false, err
 	}
 	return resp.GetDelivered(), nil
 }
 
-// SendToGameChannel posts an operator text message to the game channel of the bot
-// for the given language. delivered reports whether the connector sent it (false
-// when that bot has no channel configured).
-func (c *Client) SendToGameChannel(ctx context.Context, text, language string) (bool, error) {
-	resp, err := c.c.SendToGameChannel(ctx, &telegramv1.SendToGameChannelRequest{Text: text, Language: language})
+// SendToGameChannel posts an operator text message to the bot's game channel.
+// delivered reports whether the connector sent it (false when the bot has no
+// channel configured).
+func (c *Client) SendToGameChannel(ctx context.Context, text string) (bool, error) {
+	resp, err := c.c.SendToGameChannel(ctx, &telegramv1.SendToGameChannelRequest{Text: text})
 	if err != nil {
 		return false, err
 	}