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/inttest/payment_availability_test.go

@@ -0,0 +1,83 @@
+//go:build integration
+
+package inttest
+
+import (
+	"context"
+	"testing"
+
+	"github.com/google/uuid"
+
+	"scrabble/backend/internal/payments"
+)
+
+// TestPaymentAvailabilityKillSwitch exercises the per-rail kill switch + the per-account override
+// end-to-end against Postgres: fail-open, a disabled rail with a localized message, per-rail
+// granularity, and the allow/deny/default overrides.
+func TestPaymentAvailabilityKillSwitch(t *testing.T) {
+	ctx := context.Background()
+	svc := newPaymentsService()
+	acc := uuid.New()
+
+	// Fail-open: an untouched rail is enabled.
+	if ok, _, err := svc.CanPurchase(ctx, acc, payments.RailDirectWeb, "en"); err != nil || !ok {
+		t.Fatalf("fail-open: CanPurchase = %v/%v, want true", ok, err)
+	}
+
+	// Disable the web rail with a per-language message → blocked with that message, localized.
+	if err := svc.SetRailStatus(ctx, payments.RailDirectWeb, payments.RailAvailability{MessageRU: "Чиним", MessageEN: "Fixing"}); err != nil {
+		t.Fatalf("set rail status: %v", err)
+	}
+	if ok, reason, err := svc.CanPurchase(ctx, acc, payments.RailDirectWeb, "ru"); err != nil || ok || reason != "Чиним" {
+		t.Fatalf("disabled rail (ru): CanPurchase = %v/%q/%v, want false/Чиним", ok, reason, err)
+	}
+	if ok, reason, _ := svc.CanPurchase(ctx, acc, payments.RailDirectWeb, "en"); ok || reason != "Fixing" {
+		t.Fatalf("disabled rail (en): = %v/%q, want false/Fixing", ok, reason)
+	}
+	// Per-rail granularity: another rail stays enabled.
+	if ok, _, _ := svc.CanPurchase(ctx, acc, payments.RailDirectAndroid, "en"); !ok {
+		t.Fatalf("android rail should stay enabled")
+	}
+
+	// A per-account "allow" override bypasses the disabled rail.
+	if err := svc.SetPurchaseOverride(ctx, acc, payments.OverrideAllow); err != nil {
+		t.Fatalf("set override allow: %v", err)
+	}
+	if ok, _, _ := svc.CanPurchase(ctx, acc, payments.RailDirectWeb, "en"); !ok {
+		t.Fatalf("allow override should bypass the disabled rail")
+	}
+	if ov, err := svc.PurchaseOverrideFor(ctx, acc); err != nil || ov != payments.OverrideAllow {
+		t.Fatalf("PurchaseOverrideFor = %v/%v, want allow", ov, err)
+	}
+
+	// A "deny" override blocks even an enabled rail.
+	if err := svc.SetPurchaseOverride(ctx, acc, payments.OverrideDeny); err != nil {
+		t.Fatalf("set override deny: %v", err)
+	}
+	if ok, reason, _ := svc.CanPurchase(ctx, acc, payments.RailDirectAndroid, "en"); ok || reason == "" {
+		t.Fatalf("deny override should block the enabled android rail with a reason, got %v/%q", ok, reason)
+	}
+
+	// Clearing the override (default) restores rail-driven behaviour.
+	if err := svc.SetPurchaseOverride(ctx, acc, payments.OverrideDefault); err != nil {
+		t.Fatalf("clear override: %v", err)
+	}
+	if ov, _ := svc.PurchaseOverrideFor(ctx, acc); ov != payments.OverrideDefault {
+		t.Fatalf("after clear, override = %v, want default", ov)
+	}
+	if ok, _, _ := svc.CanPurchase(ctx, acc, payments.RailDirectAndroid, "en"); !ok {
+		t.Fatalf("after clearing override, android rail should be enabled again")
+	}
+
+	// RailStatuses reflects the stored web-rail change and fills the rest fail-open.
+	all, err := svc.RailStatuses(ctx)
+	if err != nil {
+		t.Fatalf("rail statuses: %v", err)
+	}
+	if all[payments.RailDirectWeb].Enabled {
+		t.Fatalf("web rail should read disabled")
+	}
+	if !all[payments.RailVK].Enabled {
+		t.Fatalf("untouched vk rail should read enabled")
+	}
+}