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/yookassa.go

@@ -207,13 +207,22 @@ func (c Config) GetPayment(ctx context.Context, paymentID string) (Payment, erro
 }
 
 // CreateRefund returns money for a succeeded payment and reports the created refund. idempotenceKey
-// guards against a double refund on a retry; callers pass the order id.
+// guards against a double refund on a retry; callers pass the order id. The returned refund is not
+// necessarily final — check Status before acting on it.
 func (c Config) CreateRefund(ctx context.Context, req RefundRequest, idempotenceKey string) (Refund, error) {
 	var out Refund
 	err := c.do(ctx, http.MethodPost, "/refunds", req, idempotenceKey, &out)
 	return out, err
 }
 
+// GetRefund re-reads a refund by id. As with GetPayment this is the authenticity check: a refund
+// notification is unsigned, so only the object returned here may be acted on.
+func (c Config) GetRefund(ctx context.Context, refundID string) (Refund, error) {
+	var out Refund
+	err := c.do(ctx, http.MethodGet, "/refunds/"+url.PathEscape(refundID), nil, "", &out)
+	return out, err
+}
+
 // do performs one authenticated API call and decodes the result into out. A non-2xx answer is
 // returned as an *APIError carrying YooKassa's own error object when the body holds one.
 func (c Config) do(ctx context.Context, method, path string, body any, idempotenceKey string, out any) error {