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.

ui/src/screens/Wallet.svelte

@@ -9,6 +9,7 @@
   import { isGooglePlayBuild } from '../lib/distribution';
   import { onExternalLinkClick, openExternalUrl } from '../lib/links';
   import { vkPlatform, vkShowOrderBox } from '../lib/vk';
+  import { telegramOpenInvoice } from '../lib/telegram';
   import type { Wallet, Catalog, CatalogProduct } from '../lib/model';
 
   // The Wallet section: the context-visible chip balances, the active benefits, and the storefront.
@@ -119,6 +120,14 @@
       if (context === 'vk') {
         // VK settles the payment in-app via the bridge; the chips arrive on the server callback.
         await vkShowOrderBox(order.orderId);
+      } else if (context === 'telegram') {
+        // Telegram Stars: the bot minted an invoice link (returned as redirectUrl); open it in-app.
+        // The chips are credited server-side by the forwarded payment and arrive on the live push;
+        // refresh on 'paid' as a fallback. Outside a Stars-capable client, fall back to the link.
+        const opened = telegramOpenInvoice(order.redirectUrl, (status) => {
+          if (status === 'paid') void load();
+        });
+        if (!opened) openExternalUrl(order.redirectUrl);
       } else {
         openExternalUrl(order.redirectUrl);
       }