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/postgres/jet/payments/table/table_use_schema.go

@@ -10,6 +10,7 @@ package table
 // UseSchema sets a new schema name for all generated table SQL builder types. It is recommended to invoke
 // this method only once at the beginning of the program.
 func UseSchema(schema string) {
+	AccountRisk = AccountRisk.FromSchema(schema)
 	Balances = Balances.FromSchema(schema)
 	Benefits = Benefits.FromSchema(schema)
 	CatalogAtom = CatalogAtom.FromSchema(schema)