feat(payments): reverse refunds issued outside the console

Two holes on the refund path, both found by asking what happens when a refund
does not come from our own `/_gm` button.

The merchant cabinet is a second entry point. An operator can refund there, and
such a refund never passes through our API — so the money went back while the
chips stayed credited, silently. Handle `refund.succeeded`: the refund is
re-read from the API (the notification body is no more evidence here than it is
for a payment), bound back to its order through the payment id recorded when
the payment was minted, and reversed through the same engine. It is idempotent
on (provider, refund id), so the event for a refund the console already
recorded reverses nothing twice.

The reversal engine is full-refund-only by design — it revokes exactly what the
pack funded and rejects any other amount — so a partial refund is recorded as
nothing at all and logged loudly for an operator. There is no non-arbitrary way
to decide how many chips a part-refund costs, and guessing would be worse than
asking a human.

Second hole: a refund can still be canceled while pending, and the ledger is
append-only. Recording on any non-empty refund id therefore risked revoking a
customer's chips for money that stayed with us, with no way to take the row
back. The console now records only a `succeeded` refund and tells the operator
to press again otherwise — the idempotency key returns the same refund rather
than paying twice.

Tests: unit (GetRefund, the refund notification envelope, a non-final status
surfaced to the caller); integration (a cabinet refund is reversed once and a
redelivery is a no-op, the event after a console refund changes nothing, a
partial refund records nothing, an unconfirmed refund reverses nothing, a
pending refund records nothing until it settles and then does).

The suite shares one database and the ledger dedupes refunds globally, so the
fake provider now mints a refund id per payment — a constant id made one test's
refund look like another's duplicate.

Decisions D50 (amended) and D52; the notification subscription list in the
deploy docs gains refund.succeeded.

backend/internal/yookassa/notify.go

@@ -14,8 +14,9 @@ const (
 	// EventPaymentCanceled means the payment was actively declined or abandoned; nothing is credited
 	// and the payer is told the attempt failed.
 	EventPaymentCanceled = "payment.canceled"
-	// EventRefundSucceeded reports a completed refund. Refunds here are always initiated by an
-	// operator through the API, which records them synchronously, so this event is informational.
+	// EventRefundSucceeded reports a completed refund. It matters because the merchant cabinet is a
+	// second way to issue one: a refund made there never passes through our API, so without this
+	// event the money would go back while the chips stayed credited.
 	EventRefundSucceeded = "refund.succeeded"
 )
 
@@ -58,6 +59,19 @@ func (n Notification) Payment() (Payment, error) {
 	return p, nil
 }
 
+// Refund decodes the notification's object as a refund. Use it only for refund.* events; it yields
+// the refund id to re-read, never the refund state to act on.
+func (n Notification) Refund() (Refund, error) {
+	var r Refund
+	if err := json.Unmarshal(n.Object, &r); err != nil {
+		return Refund{}, fmt.Errorf("yookassa: decode notification refund: %w", err)
+	}
+	if r.ID == "" {
+		return Refund{}, fmt.Errorf("yookassa: notification refund has no id")
+	}
+	return r, nil
+}
+
 // senderPrefixes are the address ranges YooKassa delivers notifications from
 // (https://yookassa.ru/developers/using-api/webhooks). Single addresses are expressed as /32 and
 // /128 prefixes. The list is defence in depth only — the confirming GetPayment is what actually