fix: review-loop hardening — identical-body replay, 2FA gates, webhook at-least-once, GDPR scrub

Follow-up to the comprehensive payment-system review. Fixes the issues the
review found in the initial integration, plus the rough edges it introduced.

Money-safety:
- Replay-by-key now replays the FULL original request verbatim from a stored
  square_request_snapshot, so a retained idempotency key returns the original
  payment instead of IDEMPOTENCY_KEY_REUSED (previously the row sat pending
  forever). IDEMPOTENCY_KEY_REUSED remains ambiguous (never proof of no charge).
- Dev mock mirrors real Square for unknown-key replays: ccof: saved-card
  sources are charged and rescued; spent cnon: nonces surface
  ErrReplayKeyNotRetained. (Fixes dev/prod parity divergence.)
- Webhook dedup row committed AFTER dispatch (at-least-once); FAILED till sales
  claw back gift-card funding; event-type strings match Square's real catalog.
- Expired-gift-card cancellation refunds set creditFailed (never a phantom
  'completed' refund); cancellation refunds lock all payment rows ascending.
- Sweep never rescue-completes a gift-card purchase without delivering the card.
- Tip no-client-key fallback is a deterministic count-based key under the
  booking advisory lock (retry-safe, distinct tips don't collapse).
- M-cap subtracts completed refunds, clamped to [0, total].

2FA (PSD2 SCA stand-in) for online saved-card payments:
- Full feature: status/setup/verify/disable endpoints, gating helper wired into
  all 7 saved-card charge paths (incl. BuyGiftCard + admin saved-card), account
  admin-tab settings UI, frontend gating across all payment surfaces.
- Enforcement is FAIL-CLOSED: on unless REQUIRE_2FA=false or an explicit
  mock/dev SQUARE_ENVIRONMENT; startup warning when off in a non-dev env.
- Verify is brute-force hardened (5-attempt lockout, timing-safe compare);
  plaintext codes only logged when enforcement is off (dev).
- GDPR: anonymize_user also scrubs 2FA columns and staff notes.

Infra/docs:
- nginx: /api/ response cache removed (cross-user disclosure); port 80
  redirects to HTTPS (localhost/RFC1918 exempt, end-anchored regexes); HSTS;
  separate webhook rate-limit zone.
- Schema: users 2FA columns; payments/till_sales square_source_id +
  square_request_snapshot.
- Legal docs: gift-card cooling-off, international-transfers section, tips
  policy; Gap Backlog P3 webhooks marked done; stale counts/wording corrected.
- Flaky test race fixed (t.Parallel + global mock mutation); suite 26/26
  packages green, 2,142 tests, svelte-check clean.

frontend/src/lib/components/booking/BookingFlow.svelte

@@ -139,6 +139,13 @@
 
 	const canSaveCards = $derived(canSaveCardsForRole(authStore.currentUser?.role));
 
+	// PSD2 SCA stand-in: 2FA required but not enabled blocks saved-card use
+	// and saving new cards for reuse. The new-card (nonce) path has its own
+	// SCA via Square tokenizeWithVerification.
+	const twoFactorBlocksSavedCards = $derived(
+		!!authStore.currentUser?.twoFactorRequired && !authStore.currentUser?.twoFactorEnabled
+	);
+
 	const depositCardFormValid = $derived(paymentCardSelectionValid);
 
 	// VAT registration status from public business info (via shared store)
@@ -309,6 +316,12 @@
 		isProcessingPayment = true;
 		paymentAttempted = false;
 		try {
+			// PSD2 SCA stand-in: never charge a saved card while 2FA is required
+			// but not enabled — clear any stale selection so the new-card
+			// (nonce) path is used instead.
+			if (twoFactorBlocksSavedCards && selectedPaymentMethod) {
+				selectedPaymentMethod = '';
+			}
 			await submitAndProceed();
 			if (!confirmedBooking) {
 				toast.error('Booking was not created. Please try again.');
@@ -327,7 +340,7 @@
 
 			let newCardToken: string | undefined;
 			let verificationToken: string | undefined;
-			if (selectedPaymentMethod) {
+			if (selectedPaymentMethod && !twoFactorBlocksSavedCards) {
 				// saved card — nothing to tokenize
 			} else if (paymentCardSelection) {
 				// New-card mode: tokenize once per attempt, reuse the nonce + SCA