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.

gateway/internal/transcode/transcode.go

@@ -189,6 +189,17 @@ func (r *Registry) Lookup(messageType string) (Op, bool) {
 	return op, ok
 }
 
+// DomainMessage returns the human-readable, already-localized reason a backend domain error carries
+// for the user (e.g. an operator's payment-unavailable explanation), or "" when it carries none. It
+// rides the Execute envelope's message field alongside the result code.
+func DomainMessage(err error) string {
+	var apiErr *backendclient.APIError
+	if errors.As(err, &apiErr) {
+		return apiErr.Message
+	}
+	return ""
+}
+
 // DomainCode maps an error to a stable result code to surface in the Execute
 // envelope, reporting false for an unexpected error the caller should treat as a
 // transport-level internal failure.