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.

backend/handlers/payments/refunds_test.go

@@ -531,6 +531,18 @@ func TestProcessCancellationRefund_ExpiredGiftCard_Retained(t *testing.T) {
 	if txCount != 0 {
 		t.Errorf("expected NO refund-to-expired-card transaction, got %d", txCount)
 	}
+
+	// C4 money-safety: the refund record must be 'failed' — never 'completed'.
+	// The old bug fell through the switch and recorded a completed refund even
+	// though the UPDATE gift_cards credit was skipped (no money moved).
+	var refundStatus string
+	if err := tx.QueryRow(ctx,
+		"SELECT status FROM refunds WHERE booking_id = $1", bookingID).Scan(&refundStatus); err != nil {
+		t.Fatalf("failed to query refund status: %v", err)
+	}
+	if refundStatus != "failed" {
+		t.Errorf("expected expired-card refund record status 'failed' (no money credited), got %q", refundStatus)
+	}
 }
 
 func TestProcessCancellationRefund_CashCreditsUserBalance(t *testing.T) {