feat(payments): refund engine (best-effort revoke, never negative)

The last money-intake slice: reverse a paid order best-effort, exactly once. All
refunds are admin-triggered (E7) — no rail pushes an unsolicited refund (Robokassa
via its refund API / cabinet, VK via support, Telegram via refundStarPayment), so
this ships the engine they all converge on, not a webhook.

The Refund method matches the paid order, appends a refund ledger row (idempotent on
(provider, provider_refund_id) — distinct from the fund's payment id, so both
coexist), and revokes the funded chips floored at 0 (never negative — D27,
balances_chips_chk). When the chips were already spent, the unrecoverable remainder
is recorded as a per-account loss + abuse flag in the new additive
payments.account_risk table (read by the E7 report). The refund ledger row's chip
delta is what was actually reclaimed (the ledger stays reconcilable); the full
reversal rides in the snapshot; the order stays paid.

Additive migration (a new table only) -> rollback-safe, no contour wipe. Robokassa
refund-status polling is deferred (a worker not worth it at low chargeback volume);
failed events are not wired (no rail signals a hard post-charge server decline).

Tests: integration (full revoke; revoke-after-spend = floor-0 + loss + abuse;
duplicate idempotent; unpaid-order guard). Docs: PAYMENTS(+ru) §9, PLAN (E5 -> DONE).

backend/internal/payments/store_intake.go

@@ -27,6 +27,9 @@ var (
 	// ErrAmountMismatch means the callback's paid amount or currency does not match the order's
 	// expected amount — the credit is refused (§9: verify amount after matching by order id).
 	ErrAmountMismatch = errors.New("payments: paid amount does not match the order")
+	// ErrOrderNotPaid means a refund targets an order that was never funded — there is nothing to
+	// reverse (guards against a spurious loss/abuse record on an unpaid order).
+	ErrOrderNotPaid = errors.New("payments: order is not paid")
 )
 
 // errAlreadyCredited is the internal sentinel that unwinds the fund transaction when the ledger's
@@ -34,6 +37,10 @@ var (
 // a replayed callback is a success that credits nothing.
 var errAlreadyCredited = errors.New("payments: already credited")
 
+// errAlreadyRefunded unwinds the refund transaction when the ledger idempotency index rejects a
+// duplicate refund (same provider refund id). It is not surfaced: a replayed refund reverses nothing.
+var errAlreadyRefunded = errors.New("payments: already refunded")
+
 // packInfo is a chip pack resolved for an order: the product, the chips it funds and its price in
 // the requested payment method's currency.
 type packInfo struct {
@@ -271,6 +278,103 @@ func (s *Store) fund(ctx context.Context, orderID uuid.UUID, provider, providerP
 	return outcome, nil
 }
 
+// RefundOutcome reports a refund's result: whose funded segment was reversed, the chips actually
+// clawed back (floored at 0), the unrecoverable remainder (a loss, when the chips were already
+// spent), and whether the refund was a duplicate that reversed nothing.
+type RefundOutcome struct {
+	AccountID       uuid.UUID
+	Source          Source
+	Revoked         int
+	Loss            int
+	AlreadyRefunded bool
+}
+
+// refund reverses a paid order's credit best-effort, exactly once. It matches the order (which must
+// be paid), verifies the refunded amount, then in one transaction appends a refund ledger row
+// (idempotent on the (provider, provider_payment_id) index — the refund id is distinct from the
+// fund's payment id, so the two rows coexist), revokes the funded chips floored at 0 (never
+// negative, D27/balances_chips_chk) and, when chips were already spent, records the unrecoverable
+// remainder as a per-account loss and flips the abuse flag. A duplicate refund returns
+// AlreadyRefunded with no second reversal. The ledger row's chipsDelta is what is actually
+// reclaimed; the full reversal (money, original chips, loss) rides in its snapshot for the report.
+func (s *Store) refund(ctx context.Context, orderID uuid.UUID, provider, providerRefundID string, refunded Money, now time.Time) (RefundOutcome, error) {
+	ord, err := s.orderByID(ctx, orderID)
+	if err != nil {
+		return RefundOutcome{}, err
+	}
+	if ord.status != "paid" {
+		return RefundOutcome{}, ErrOrderNotPaid
+	}
+	if refunded.Currency() != Currency(ord.currency) || refunded.Minor() != ord.expectedAmount {
+		return RefundOutcome{}, ErrAmountMismatch
+	}
+	chips, title, err := s.packForCredit(ctx, ord.productID)
+	if err != nil {
+		return RefundOutcome{}, err
+	}
+
+	src := Source(ord.origin)
+	outcome := RefundOutcome{AccountID: ord.accountID, Source: src}
+	pv, pr := provider, providerRefundID
+	productID := ord.productID
+	oid := orderID
+	err = withTx(ctx, s.db, func(tx *sql.Tx) error {
+		// Lock the funded segment and read what is left; a spent balance floors the reversal at 0.
+		var avail int
+		e := tx.QueryRowContext(ctx,
+			`SELECT chips FROM payments.balances WHERE account_id = $1 AND source = $2 FOR UPDATE`,
+			ord.accountID, string(src)).Scan(&avail)
+		switch {
+		case errors.Is(e, sql.ErrNoRows):
+			avail = 0
+		case e != nil:
+			return fmt.Errorf("payments: read balance for refund: %w", e)
+		}
+		revoked := min(chips, avail)
+		loss := chips - revoked
+		outcome.Revoked, outcome.Loss = revoked, loss
+
+		snapshot, e := marshalRefundSnapshot(ord.productID, title, chips, revoked, loss, refunded, providerRefundID)
+		if e != nil {
+			return e
+		}
+		if e := insertLedgerTx(ctx, tx, ord.accountID, "refund", &src, &src, -revoked, &productID, &oid, &pv, &pr, snapshot, now); e != nil {
+			if isUniqueViolation(e) {
+				outcome.AlreadyRefunded = true
+				return errAlreadyRefunded
+			}
+			return e
+		}
+		if revoked > 0 {
+			if _, e := tx.ExecContext(ctx,
+				`UPDATE payments.balances SET chips = chips - $3, updated_at = now()
+				 WHERE account_id = $1 AND source = $2`,
+				ord.accountID, string(src), revoked); e != nil {
+				return fmt.Errorf("payments: revoke chips %s: %w", src, e)
+			}
+		}
+		if loss > 0 {
+			if _, e := tx.ExecContext(ctx,
+				`INSERT INTO payments.account_risk (account_id, abuse, loss_chips, updated_at)
+				 VALUES ($1, true, $2, now())
+				 ON CONFLICT (account_id) DO UPDATE
+				   SET abuse = true, loss_chips = payments.account_risk.loss_chips + EXCLUDED.loss_chips, updated_at = now()`,
+				ord.accountID, int64(loss)); e != nil {
+				return fmt.Errorf("payments: record refund loss: %w", e)
+			}
+		}
+		return nil
+	})
+	if err != nil {
+		if errors.Is(err, errAlreadyRefunded) {
+			return outcome, nil
+		}
+		return RefundOutcome{}, err
+	}
+	s.cache.invalidate(ord.accountID)
+	return outcome, nil
+}
+
 // insertPaymentEvent appends an undispatched lifecycle event (succeeded/failed/refunded) for the
 // dispatcher to deliver. orderID and payload (a jsonb detail blob) are optional.
 func (s *Store) insertPaymentEvent(ctx context.Context, accountID uuid.UUID, orderID *uuid.UUID, eventType string, payload []byte, now time.Time) error {
@@ -373,6 +477,27 @@ func marshalFundSnapshot(productID uuid.UUID, title string, chips int, paid Mone
 	return b, nil
 }
 
+// marshalRefundSnapshot records the full reversal on the refund ledger row: the pack, the original
+// funded chips, how many were actually reclaimed, the unrecoverable loss (already spent), the money
+// refunded and the provider refund id — so the ledger stays reconcilable against the balance
+// (chipsDelta = revoked) while the report still sees the whole reversal (§7/D27/D34).
+func marshalRefundSnapshot(productID uuid.UUID, title string, chips, revoked, loss int, refunded Money, refundID string) ([]byte, error) {
+	b, err := json.Marshal(struct {
+		ProductID string `json:"product_id"`
+		Title     string `json:"title,omitempty"`
+		Chips     int    `json:"chips"`
+		Revoked   int    `json:"revoked"`
+		Loss      int    `json:"loss"`
+		Amount    int64  `json:"amount_minor"`
+		Currency  string `json:"currency"`
+		RefundID  string `json:"refund_id"`
+	}{productID.String(), title, chips, revoked, loss, refunded.Minor(), string(refunded.Currency()), refundID})
+	if err != nil {
+		return nil, fmt.Errorf("payments: marshal refund snapshot: %w", err)
+	}
+	return b, nil
+}
+
 // isUniqueViolation reports whether err is a PostgreSQL unique-constraint violation (SQLSTATE
 // 23505) — here, a duplicate provider callback hitting the ledger idempotency index.
 func isUniqueViolation(err error) bool {