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/server/handlers_admin_catalog.go

@@ -51,7 +51,16 @@ func (s *Server) consoleCatalog(c *gin.Context) {
 		s.consoleError(c, err)
 		return
 	}
+	rails, err := s.payments.RailStatuses(c.Request.Context())
+	if err != nil {
+		s.consoleError(c, err)
+		return
+	}
 	view := adminconsole.CatalogView{ShowAll: showAll, RewardPayout: payout, RewardDailyCap: daily, RewardHourlyCap: hourly}
+	for _, rail := range payments.KnownRails {
+		a := rails[rail]
+		view.Rails = append(view.Rails, adminconsole.RailStatusRow{Rail: rail, Enabled: a.Enabled, MessageRU: a.MessageRU, MessageEN: a.MessageEN})
+	}
 	for _, p := range products {
 		view.Products = append(view.Products, catalogRow(p))
 	}
@@ -72,6 +81,23 @@ func (s *Server) consoleSetReward(c *gin.Context) {
 	s.renderConsoleMessage(c, "Saved", "the rewarded-ads config was updated", catalogBack)
 }
 
+// consoleSetRailStatus toggles a payment rail's operational kill switch and its user-facing
+// off-message (per language) from the catalog page's payment-availability form. An unchecked box
+// disables the rail; an unknown rail is refused by the service.
+func (s *Server) consoleSetRailStatus(c *gin.Context) {
+	rail := strings.TrimSpace(c.PostForm("rail"))
+	a := payments.RailAvailability{
+		Enabled:   c.PostForm("enabled") == "on",
+		MessageRU: strings.TrimSpace(c.PostForm("message_ru")),
+		MessageEN: strings.TrimSpace(c.PostForm("message_en")),
+	}
+	if err := s.payments.SetRailStatus(c.Request.Context(), rail, a); err != nil {
+		s.renderConsoleMessage(c, "Update failed", err.Error(), catalogBack)
+		return
+	}
+	s.renderConsoleMessage(c, "Saved", "payment availability was updated", catalogBack)
+}
+
 // catalogRow projects a product into its list row.
 func catalogRow(p payments.AdminProduct) adminconsole.ProductRow {
 	row := adminconsole.ProductRow{ID: p.ID.String(), Title: p.Title, Active: p.Active, Transacted: p.Transacted}