feat(payments): Telegram Stars payment rail

Accept real money via Telegram Stars (XTR) — the third intake rail
alongside Robokassa (direct) and VK Votes.

Only the bot reaches Telegram, so the rail funnels through the reverse
mTLS bot-link:
- the gateway mints the invoice on a CreateInvoice command (the bot
  calls createInvoiceLink, XTR; the link goes to WebApp.openInvoice);
- the bot gates each pre_checkout_query via a ValidatePreCheckout unary
  (the order must exist, be still creditable and not already paid — the
  reusable-invoice double-pay guard; the decline reason is localised to
  the order account's language);
- a completed successful_payment is queued in a durable pure-Go SQLite
  outbox and forwarded via a ForwardPayment unary, credited once
  (idempotent on telegram_payment_charge_id, honours an expired order),
  re-driven on restart and every 30s.

The rail is wired by TELEGRAM_STARS_OUTBOX_DIR (default /data) but stays
inert until a chip pack carries an XTR price, so seeding a Stars price in
the admin is the go-live.

Tests: backend integration (order->forward->credit once, duplicate,
pre_checkout gate) + bot outbox unit (idempotent, restart re-drive) +
executor createInvoice. Docs: PAYMENTS(+ru) §9, ARCHITECTURE, the
platform/telegram README, PLAN.

deploy/docker-compose.bot.yml

@@ -28,6 +28,10 @@ services:
       # the manage-topics and delete-messages rights.
       TELEGRAM_SUPPORT_CHAT_ID: ${TELEGRAM_SUPPORT_CHAT_ID:-}
       TELEGRAM_SUPPORT_STATE_DIR: /data
+      # The Telegram Stars payment outbox (shares the bot-state volume). A writable dir enables the
+      # Stars rail; empty disables it. The rail stays inert until a chip pack carries an XTR (Stars)
+      # price, so it is safe to leave on — seeding a Stars price in the admin is the real go-live.
+      TELEGRAM_STARS_OUTBOX_DIR: ${TELEGRAM_STARS_OUTBOX_DIR:-/data}
       TELEGRAM_PROMO_BOT_TOKEN: ${TELEGRAM_PROMO_BOT_TOKEN:-}
       TELEGRAM_BOT_USERNAME: ${TELEGRAM_BOT_USERNAME:-}
       TELEGRAM_BOT_LINK: ${TELEGRAM_BOT_LINK:-}

deploy/docker-compose.yml

@@ -385,6 +385,10 @@ services:
       # set the bot must be an admin there with the manage-topics and delete-messages rights.
       TELEGRAM_SUPPORT_CHAT_ID: ${TELEGRAM_SUPPORT_CHAT_ID:-}
       TELEGRAM_SUPPORT_STATE_DIR: /data
+      # The Telegram Stars payment outbox (shares the bot-state volume). A writable dir enables the
+      # Stars rail (pre_checkout + successful_payment + the durable outbox); empty disables it. The
+      # rail stays inert until a chip pack carries an XTR (Stars) price, so it is safe to leave on.
+      TELEGRAM_STARS_OUTBOX_DIR: ${TELEGRAM_STARS_OUTBOX_DIR:-/data}
       # The optional standalone promo bot (its own token) answering /start with a button
       # into the main bot's app. Empty disables it; when set it needs the main bot's
       # @username and the Mini App link (reused from the UI's VITE_TELEGRAM_LINK).