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.

platform/telegram/README.md

@@ -83,7 +83,10 @@ Telegram identity to an account from a browser. Both map a rejection to gRPC
   the main bot's direct link (`TELEGRAM_BOT_LINK`, the same link the UI uses) with
   `?startapp` — a `web_app` button would launch under the promo bot's identity (its token
   would sign the initData), which the main bot's validator rejects. It is fully
-  self-contained: no bot-link, no gateway, no game.
+  self-contained: no bot-link, no gateway, no game. Its `?startapp` payload
+  (`TELEGRAM_PROMO_START_PARAM`, default `verudit_ru-scrabble_en`) is a variant-seed deep
+  link the backend decodes to seed a brand-new user's variant preferences (English Scrabble
+  alongside the default Erudit).
 - **Rate limiting.** Outbound sends are throttled (`TELEGRAM_SEND_RATE_PER_SECOND`,
   default 25) to respect the Bot API flood limits.
 
@@ -151,6 +154,7 @@ Bot (`cmd/bot`):
 | `TELEGRAM_PROMO_BOT_TOKEN` | — | the optional standalone promo bot's token; empty disables it |
 | `TELEGRAM_BOT_USERNAME` | — | the main bot's @username without the @ (promo message); required when the promo bot runs |
 | `TELEGRAM_BOT_LINK` | — | the main bot's Mini App link for the promo button (the UI's `VITE_TELEGRAM_LINK`); required when the promo bot runs |
+| `TELEGRAM_PROMO_START_PARAM` | `verudit_ru-scrabble_en` | the promo button's `startapp` payload — a variant-seed deep link the backend decodes to seed a brand-new user's variant preferences; empty forwards the user's own `/start` payload |
 | `TELEGRAM_OWNS_UPDATES` | `true` | run the exclusive `getUpdates` long-poll (one bot per token) |
 | `TELEGRAM_SEND_RATE_PER_SECOND` | `25` | outbound Bot API send cap (0 disables) |
 | `TELEGRAM_INSTANCE_ID` | hostname | bot identity reported to the gateway |