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/model/account_risk.go

@@ -0,0 +1,20 @@
+//
+// Code generated by go-jet DO NOT EDIT.
+//
+// WARNING: Changes to this file may cause incorrect behavior
+// and will be lost if the code is regenerated
+//
+
+package model
+
+import (
+	"github.com/google/uuid"
+	"time"
+)
+
+type AccountRisk struct {
+	AccountID uuid.UUID `sql:"primary_key"`
+	Abuse     bool
+	LossChips int64
+	UpdatedAt time.Time
+}

backend/internal/postgres/jet/payments/table/account_risk.go

@@ -0,0 +1,87 @@
+//
+// Code generated by go-jet DO NOT EDIT.
+//
+// WARNING: Changes to this file may cause incorrect behavior
+// and will be lost if the code is regenerated
+//
+
+package table
+
+import (
+	"github.com/go-jet/jet/v2/postgres"
+)
+
+var AccountRisk = newAccountRiskTable("payments", "account_risk", "")
+
+type accountRiskTable struct {
+	postgres.Table
+
+	// Columns
+	AccountID postgres.ColumnString
+	Abuse     postgres.ColumnBool
+	LossChips postgres.ColumnInteger
+	UpdatedAt postgres.ColumnTimestampz
+
+	AllColumns     postgres.ColumnList
+	MutableColumns postgres.ColumnList
+	DefaultColumns postgres.ColumnList
+}
+
+type AccountRiskTable struct {
+	accountRiskTable
+
+	EXCLUDED accountRiskTable
+}
+
+// AS creates new AccountRiskTable with assigned alias
+func (a AccountRiskTable) AS(alias string) *AccountRiskTable {
+	return newAccountRiskTable(a.SchemaName(), a.TableName(), alias)
+}
+
+// Schema creates new AccountRiskTable with assigned schema name
+func (a AccountRiskTable) FromSchema(schemaName string) *AccountRiskTable {
+	return newAccountRiskTable(schemaName, a.TableName(), a.Alias())
+}
+
+// WithPrefix creates new AccountRiskTable with assigned table prefix
+func (a AccountRiskTable) WithPrefix(prefix string) *AccountRiskTable {
+	return newAccountRiskTable(a.SchemaName(), prefix+a.TableName(), a.TableName())
+}
+
+// WithSuffix creates new AccountRiskTable with assigned table suffix
+func (a AccountRiskTable) WithSuffix(suffix string) *AccountRiskTable {
+	return newAccountRiskTable(a.SchemaName(), a.TableName()+suffix, a.TableName())
+}
+
+func newAccountRiskTable(schemaName, tableName, alias string) *AccountRiskTable {
+	return &AccountRiskTable{
+		accountRiskTable: newAccountRiskTableImpl(schemaName, tableName, alias),
+		EXCLUDED:         newAccountRiskTableImpl("", "excluded", ""),
+	}
+}
+
+func newAccountRiskTableImpl(schemaName, tableName, alias string) accountRiskTable {
+	var (
+		AccountIDColumn = postgres.StringColumn("account_id")
+		AbuseColumn     = postgres.BoolColumn("abuse")
+		LossChipsColumn = postgres.IntegerColumn("loss_chips")
+		UpdatedAtColumn = postgres.TimestampzColumn("updated_at")
+		allColumns      = postgres.ColumnList{AccountIDColumn, AbuseColumn, LossChipsColumn, UpdatedAtColumn}
+		mutableColumns  = postgres.ColumnList{AbuseColumn, LossChipsColumn, UpdatedAtColumn}
+		defaultColumns  = postgres.ColumnList{AbuseColumn, LossChipsColumn, UpdatedAtColumn}
+	)
+
+	return accountRiskTable{
+		Table: postgres.NewTable(schemaName, tableName, alias, allColumns...),
+
+		//Columns
+		AccountID: AccountIDColumn,
+		Abuse:     AbuseColumn,
+		LossChips: LossChipsColumn,
+		UpdatedAt: UpdatedAtColumn,
+
+		AllColumns:     allColumns,
+		MutableColumns: mutableColumns,
+		DefaultColumns: defaultColumns,
+	}
+}

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)

backend/internal/postgres/migrations/00012_account_risk.sql

@@ -0,0 +1,25 @@
+-- Per-account payment risk: the loss and abuse signal an external/admin refund leaves
+-- behind when the refunded chips were already spent. A refund revokes chips best-effort
+-- and never drives a balance negative (D27, balances_chips_chk); the unrecoverable
+-- remainder is a recorded loss and flips an abuse flag the /_gm financial report reads
+-- (D40, E7). Mutable per-account state (upserted on each such refund), so — unlike the
+-- ledger — it carries no append-only trigger. Additive: a new table only, so goose
+-- applies it forward with no rewrite of existing data (the contour is not wiped). The
+-- payments role inherits ALL on it via the schema default privileges set in 00010.
+-- +goose Up
+
+CREATE TABLE payments.account_risk (
+    account_id uuid NOT NULL,
+    -- abuse flips true the first time a refund cannot fully reclaim its chips (spent).
+    abuse boolean DEFAULT false NOT NULL,
+    -- loss_chips accumulates the unrecoverable chips across such refunds (bigint: an
+    -- accumulator, mapped to int64 by go-jet — not the numeric->float64 trap).
+    loss_chips bigint DEFAULT 0 NOT NULL,
+    updated_at timestamp with time zone DEFAULT now() NOT NULL,
+    CONSTRAINT account_risk_pkey PRIMARY KEY (account_id),
+    CONSTRAINT account_risk_loss_chips_chk CHECK ((loss_chips >= 0))
+);
+
+-- +goose Down
+
+DROP TABLE IF EXISTS payments.account_risk;