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.

pkg/proto/botlink/v1/botlink.proto

@@ -27,6 +27,24 @@ service BotLink {
   // same mTLS channel when a user joins the chat, to decide whether to grant the
   // write permission. Delivery of the answer is request/response (not best-effort).
   rpc ResolveChatEligibility(ChatEligibilityRequest) returns (ChatEligibilityResponse);
+
+  // ValidatePreCheckout answers whether a Telegram Stars pre_checkout_query may be
+  // approved before any star is charged: the order in the invoice payload exists, is
+  // still creditable (pending or an honoured-expired order, never one already paid),
+  // and its amount and currency match the invoice. The bot calls it on every
+  // pre_checkout_query and approves only on ok; a not-ok answer or a channel failure
+  // declines the charge (fail-closed). Reusable Stars invoice links make this gate the
+  // one place a repeat payment is stopped before money moves. Request/response.
+  rpc ValidatePreCheckout(PreCheckoutRequest) returns (PreCheckoutResponse);
+
+  // ForwardPayment delivers a completed Telegram Stars payment from the bot's durable
+  // outbox to the gateway for crediting. The bot calls it (retrying until it gets a
+  // response) after Telegram confirms the payment; the gateway forwards it to the
+  // backend intake, which credits the order once, idempotent on
+  // telegram_payment_charge_id. A response means the payment was durably handled
+  // (credited, or recorded as unmatched) and the bot may forget the outbox row; a
+  // transport error leaves the row for a later retry. Request/response.
+  rpc ForwardPayment(ForwardPaymentRequest) returns (ForwardPaymentResponse);
 }
 
 // FromBot is a message the bot sends to the gateway: the opening Hello, then one
@@ -61,17 +79,21 @@ message Command {
     scrabble.telegram.v1.SendToUserRequest send_to_user = 3;
     scrabble.telegram.v1.SendToGameChannelRequest send_to_channel = 4;
     ChatGateCommand chat_gate = 5;
+    CreateInvoiceCommand create_invoice = 6;
   }
 }
 
 // Ack reports the outcome of the Command with command_id. delivered mirrors the
 // connector delivery semantics (false when the kind is not rendered out-of-app, the
 // user never started the bot, or no channel is configured); error carries an
-// unexpected transport/render failure, distinct from a clean not-delivered.
+// unexpected transport/render failure, distinct from a clean not-delivered. result
+// carries a command's return value when it has one (the created invoice link for a
+// create_invoice command); it is empty otherwise.
 message Ack {
   string command_id = 1;
   bool delivered = 2;
   string error = 3;
+  string result = 4;
 }
 
 // ChatGateCommand sets a Telegram user's write access in the moderated discussion
@@ -99,3 +121,48 @@ message ChatEligibilityResponse {
   bool registered = 1;
   bool eligible = 2;
 }
+
+// CreateInvoiceCommand asks the bot to mint a Telegram Stars invoice link for a
+// pending order (createInvoiceLink in XTR). payload is the order id, which Telegram
+// echoes back in the pre_checkout_query and the successful_payment; amount is the
+// price in whole stars; title and description are shown on the invoice. The bot
+// returns the link in its Ack result.
+message CreateInvoiceCommand {
+  string title = 1;
+  string description = 2;
+  string payload = 3;
+  int64 amount = 4;
+}
+
+// PreCheckoutRequest asks whether a Stars pre_checkout_query for order_id at amount
+// (whole stars) in currency may be approved before the charge.
+message PreCheckoutRequest {
+  string order_id = 1;
+  int64 amount = 2;
+  string currency = 3;
+}
+
+// PreCheckoutResponse is the approval answer. ok approves the charge; reason carries a
+// short user-facing decline message when ok is false.
+message PreCheckoutResponse {
+  bool ok = 1;
+  string reason = 2;
+}
+
+// ForwardPaymentRequest carries a completed Stars payment: order_id from the invoice
+// payload, the Telegram charge id (the idempotency key), the amount in whole stars,
+// and the payer's Telegram user id.
+message ForwardPaymentRequest {
+  string order_id = 1;
+  string telegram_payment_charge_id = 2;
+  int64 amount = 3;
+  int64 telegram_user_id = 4;
+}
+
+// ForwardPaymentResponse reports the durable outcome. credited is true when the order
+// was credited (or already had been); false means the payment was recorded but could
+// not be matched to a creditable order (an operator follows up). Either way the bot
+// may forget the outbox row — only a transport error triggers a retry.
+message ForwardPaymentResponse {
+  bool credited = 1;
+}