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/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;