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/service_intake.go

@@ -81,6 +81,17 @@ func (s *Service) Fund(ctx context.Context, orderID uuid.UUID, provider, provide
 	return s.store.fund(ctx, orderID, provider, providerPaymentID, paid, s.clock())
 }
 
+// Refund reverses a paid order's credit best-effort, exactly once — for an external refund or an
+// admin-initiated one (E7). It revokes the funded chips floored at 0 (never negative, D27), records
+// any unrecoverable remainder as a per-account loss and abuse flag, and appends a refund ledger row
+// idempotent on (provider, providerRefundID) — distinct from the fund's payment id. A duplicate
+// refund returns AlreadyRefunded. The caller records the refunded payment event and performs any
+// provider-side money-back (the rails have no unsolicited refund push: Robokassa via its refund API
+// / cabinet, VK via support, Telegram via refundStarPayment — all admin-triggered).
+func (s *Service) Refund(ctx context.Context, orderID uuid.UUID, provider, providerRefundID string, refunded Money) (RefundOutcome, error) {
+	return s.store.refund(ctx, orderID, provider, providerRefundID, refunded, s.clock())
+}
+
 // Pre-checkout decline reason codes. They are language-neutral: the transport layer localises them
 // to the order account's preferred language before showing the payer (the reason is displayed in the
 // Telegram payment sheet).