fix: adversarial review round — replay-rescue double-charge, discount credit, 2FA/per-IP limits, snapshot encryption, refund reconciliation, VAT, frontend parity, tests+docs

Addresses the adversarial fresh-eyes audit (findings A1-A20) plus review-round fixes:
- CRITICAL A1: replay-by-key rescue cross-checks replayed CreatedAt; ccof blind-fail leaves pending with CRITICAL + notification instead of clawing back
- A2/A3/A4: till idempotency key restored to unconditional hash; tip rejected in CreateBookingPayment; campaign discount now reduces the charged amount (deposit credit)
- A5: admin notifications on blind-fail, manual-refund re-arm, cap-stranded charge-group, webhook FAILED/REJECTED refunds
- A6/A10: BuyGiftCard idempotency user-scoped; gift-card slot scan advances past failed rows
- A7/A14/A15: 2FA user+IP limiter, SNAPSHOT_ENC_KEY startup validation, accurate pepper/log-delivery docs
- A8/A9: snapshot encryption on all write+reuse sites; MPV->SPV effective voucher type (single VAT point)
- A11/A12/A13/A16: amount-aware refund reconciliation; completed-booking refund re-validation; till retry dedup; PaymentWasRefunded on SquareClient interface
- A17/A18/A19/A20: CI runs npm test; confirm_overflow_tip frontend dialog; unknown-event admin notification; mock token redaction
- M7 ConfirmOverflowTip, M9 snapshot encryption, C1 discount ordering regression test
- Frontend vitest framework (41 tests), backend coverage for fixed functions, docs corrected (2,269 tests, SUPPORT_EMAIL tokens, resolution status)

All 25 backend packages pass; frontend 41/41; build + env-docs green.

backend/mw/ratelimit_dev_test.go

@@ -0,0 +1,48 @@
+//go:build dev && !test
+
+package mw
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+// The dev build (ratelimit_dev.go) swaps the real rate limiters for no-op
+// passthroughs so the dev seed (11+ logins, bursty seeding traffic) never
+// trips a limiter. These tests pin that contract for a pure dev build
+// (-tags dev, no test tag): every request passes through untouched.
+
+func TestDevBuild_RateLimit_PassesEverythingThrough(t *testing.T) {
+	handler := RateLimit(2, time.Minute)(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	for i := 0; i < 25; i++ {
+		req := httptest.NewRequest(http.MethodGet, "/", nil)
+		req.RemoteAddr = "192.0.2.1:1234"
+		w := httptest.NewRecorder()
+		handler.ServeHTTP(w, req)
+		if w.Code != http.StatusOK {
+			t.Fatalf("dev no-op RateLimit blocked request %d with %d", i+1, w.Code)
+		}
+	}
+}
+
+func TestDevBuild_ProgressiveRateLimit_PassesEverythingThrough(t *testing.T) {
+	handler := ProgressiveRateLimit(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	for i := 0; i < 100; i++ {
+		req := httptest.NewRequest(http.MethodGet, "/", nil)
+		req.RemoteAddr = "192.0.2.1:1234"
+		w := httptest.NewRecorder()
+		handler.ServeHTTP(w, req)
+		if w.Code != http.StatusOK {
+			t.Fatalf("dev no-op ProgressiveRateLimit blocked request %d with %d", i+1, w.Code)
+		}
+		if got := w.Header().Get("X-RateLimit-Delay"); got != "" {
+			t.Fatalf("dev no-op ProgressiveRateLimit set X-RateLimit-Delay on request %d: %q", i+1, got)
+		}
+	}
+}