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_console.go

@@ -61,6 +61,7 @@ func (s *Server) registerConsole(router *gin.Engine) {
 	gm.POST("/users/:id/grant", s.consoleGrant)
 	gm.POST("/users/:id/grant-product", s.consoleGrantProduct)
 	gm.POST("/users/:id/refund", s.consoleRefund)
+	gm.POST("/users/:id/purchase-override", s.consoleSetPurchaseOverride)
 	gm.POST("/users/:id/delete", s.consoleDeleteUser)
 	gm.GET("/reasons", s.consoleReasons)
 	gm.POST("/reasons", s.consoleCreateReason)
@@ -113,6 +114,7 @@ func (s *Server) registerConsole(router *gin.Engine) {
 		gm.GET("/catalog", s.consoleCatalog)
 		gm.POST("/catalog", s.consoleCreateProduct)
 		gm.POST("/catalog/reward", s.consoleSetReward)
+		gm.POST("/catalog/rail-status", s.consoleSetRailStatus)
 		gm.GET("/catalog/:id", s.consoleCatalogDetail)
 		gm.POST("/catalog/:id", s.consoleUpdateProduct)
 		gm.POST("/catalog/:id/archive", s.consoleArchiveProduct)
@@ -461,6 +463,9 @@ func (s *Server) consoleUserDetail(c *gin.Context) {
 			s.log.Warn("console: account statement failed", zap.String("account", id.String()), zap.Error(err))
 		}
 		view.Grant = s.grantForm(ctx)
+		if ov, oerr := s.payments.PurchaseOverrideFor(ctx, id); oerr == nil {
+			view.PurchaseOverride = overrideName(ov)
+		}
 	}
 	s.renderConsole(c, "user_detail", "users", acc.DisplayName, view)
 }
@@ -489,6 +494,47 @@ func financeView(stmt payments.Statement) adminconsole.FinanceView {
 	return fv
 }
 
+// overrideName renders a purchase override as the form/select value ("default"/"allow"/"deny").
+func overrideName(ov payments.PurchaseOverride) string {
+	switch ov {
+	case payments.OverrideAllow:
+		return "allow"
+	case payments.OverrideDeny:
+		return "deny"
+	default:
+		return "default"
+	}
+}
+
+// consoleSetPurchaseOverride sets or clears an account's per-account purchase override (allow / deny
+// / default) from the user card. "allow" bypasses only the operational rail switch, never the
+// security gates; "default" clears the override (deletes the row).
+func (s *Server) consoleSetPurchaseOverride(c *gin.Context) {
+	id, ok := s.consoleUUID(c, "/_gm/users")
+	if !ok {
+		return
+	}
+	back := "/_gm/users/" + id.String()
+	if s.payments == nil {
+		s.renderConsoleMessage(c, "Unavailable", "payments are not configured", back)
+		return
+	}
+	var ov payments.PurchaseOverride
+	switch c.PostForm("override") {
+	case "allow":
+		ov = payments.OverrideAllow
+	case "deny":
+		ov = payments.OverrideDeny
+	default:
+		ov = payments.OverrideDefault
+	}
+	if err := s.payments.SetPurchaseOverride(c.Request.Context(), id, ov); err != nil {
+		s.renderConsoleMessage(c, "Update failed", err.Error(), back)
+		return
+	}
+	s.renderConsoleMessage(c, "Saved", "the purchase override was updated", back)
+}
+
 // relationRows maps the social graph entries to the cross-linked, date-formatted rows the
 // user card renders.
 func relationRows(rels []social.AdminRelation) []adminconsole.RelationRow {