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.

docs/PAYMENTS.md

@@ -66,6 +66,12 @@ Standalone apps (Android/iOS) sign in by email only, so a direct purchase always
 anchor (D43). Fiscalization (54-ФЗ via the Robokassa cabinet under one ИП) is a single source
 regardless of the number of shops (D41).
 
+**Payment availability kill switch (D45/D46).** An operator can disable purchases on a rail/channel
+(`direct:web` / `direct:android` / `vk` / `telegram`) or for one account, live from `/_gm`, and the
+user sees a localized reason on their next attempt (`payment_unavailable`, carried on the additive
+`ExecuteResponse.message`). **Fail-open:** a rail with no status row is enabled. A per-account
+`allow` override bypasses only this operational switch, never the security gates (D46).
+
 ## 3. Three operations, kept distinct
 
 Do not conflate these — they use different keys: