feat(payments): live payment-availability kill switch + per-account override

Let an operator disable purchases live from the admin — a whole rail/channel or
one account — and show the user a localized reason on their next attempt, so a
provider outage or a misconfig is explained instead of a silent dead button.

- rail kill switch (payments.rail_status, per rail direct:web / direct:android /
  vk / telegram): enabled + a per-language message, edited on the catalog page.
  Fail-open — a rail with no row stays enabled, so payments are never accidentally
  killed. The intake gate (CanPurchase in handleWalletOrder, before the order)
  returns payment_unavailable + the localized message, orthogonal to the security
  gates.
- per-account override (payments.account_payment_override, a row only for
  non-default): allow / deny / default, edited on the user card. "allow" bypasses
  ONLY the ops rail switch, never the security gates (trusted platform, the email
  anchor, the VK-iOS freeze, the min client version).
- wire: an additive ExecuteResponse.message envelope field (frozen-contract-safe);
  the gateway forwards a backend domain-error message; the client shows it on a
  payment_unavailable buy attempt.
- admin: rail toggles on the catalog page, the override control on the user card.
- tests: the pure gate (unit, TDD), the store + gate + override end-to-end
  (integration, migration 00016), the client (svelte-check / vitest).
- docs: PAYMENTS (+ru), the decisions log (D45/D46). Fiscalization stays
  cabinet-side (owner decision) — no itemized-receipt code.

Contour-safe: additive migration (two new tables, no wipe), the wire add is
additive, and fail-open so nothing is disabled until an operator acts.

backend/internal/postgres/migrations/00016_payment_availability.sql

@@ -0,0 +1,26 @@
+-- Payment availability controls (D45/D46): a per-rail operational kill switch with a localized message,
+-- and a per-account purchase override. Both let an operator disable payments live from the admin —
+-- a whole rail/channel, or one account — and explain why to the user on a purchase attempt. Additive
+-- (new tables) — applies forward via goose with no data rewrite (no contour wipe); an image rollback
+-- ignores both tables. Fail-open by design: a rail with no row is enabled; an account with no row
+-- follows the rail switch.
+-- +goose Up
+
+CREATE TABLE payments.rail_status (
+    rail text PRIMARY KEY,
+    enabled boolean NOT NULL DEFAULT true,
+    message_ru text NOT NULL DEFAULT '',
+    message_en text NOT NULL DEFAULT '',
+    updated_at timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE TABLE payments.account_payment_override (
+    account_id uuid PRIMARY KEY,
+    allow boolean NOT NULL,
+    updated_at timestamptz NOT NULL DEFAULT now()
+);
+
+-- +goose Down
+
+DROP TABLE payments.account_payment_override;
+DROP TABLE payments.rail_status;