feat(telegram): promo deep-link seeds English Scrabble for new users

The promo bot button carries a configurable variant-seed start-param (default verudit_ru-scrabble_en). The gateway parses start_param from the validated initData and forwards it; the backend, on first contact only, seeds the new account variant_preferences from it (English Scrabble alongside the default Erudit).

No schema change (the scrabble_en CHECK is already in the baseline) and the gateway<->backend REST field is additive, so the rolling deploy is safe in either order. TELEGRAM_PROMO_START_PARAM configures the payload (empty forwards the user own /start payload). Covered by account unit tests, a gateway transcode test, and an integration test asserting new-only seeding.

gateway/internal/backendclient/api.go

@@ -184,10 +184,10 @@ 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
-// its time zone from browserTz (the client's detected "±HH:MM" UTC offset; first
-// contact only).
-func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, username, firstName, browserTz string) (SessionResp, error) {
+// brand-new account's display name and language from the validated launch fields, its
+// time zone from browserTz (the client's detected "±HH:MM" UTC offset) and, from the
+// validated launch deep-link startParam, its variant preferences (first contact only).
+func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, username, firstName, browserTz, startParam string) (SessionResp, error) {
 	var out SessionResp
 	err := c.do(ctx, http.MethodPost, "/api/v1/internal/sessions/telegram", "", "",
 		map[string]string{
@@ -196,6 +196,7 @@ func (c *Client) TelegramAuth(ctx context.Context, externalID, languageCode, use
 			"username":      username,
 			"first_name":    firstName,
 			"browser_tz":    browserTz,
+			"start_param":   startParam,
 		}, &out)
 	return out, err
 }