fix: review round 6 — P0 deposit charge, idempotency rotation, dev-safety guard, 2FA/webhook hardening

Sixth fresh-eyes review pass (5 agents: goal, QA, code-quality, security,
context-mining). QA FAILED the deposit-required new-card flow; the P0 root
cause was backend + frontend, now fixed. All 20 packages green.

P0 money-safety:
- Deposit-required bookings now actually charge the deposit on new-card
  payment. Two-part fix: (1) CreateBookingHandler re-reads the
  trigger-maintained total_amount/total_duration_minutes from the DB after the
  booking_services insert (the INSERT..RETURNING row predates the recalc
  trigger, so TotalAmount serialized as 0 and DepositPaid computed TRUE on an
  unpaid booking — the frontend gate trusted deposit_paid:true, never charged,
  and confirmed the booking with zero payment rows); (2) BookingFlow.svelte
  gates the confirmation view on depositPaid and guards against re-creating a
  booking on retry. Regression test
  TestBookings_Create_DepositPaidFalseOnUnpaidBooking.

Payments (idempotency + money):
- deriveBookingPaymentIdempotencyKey: no-client-key fallback now advances a
  sequence for repeatable types (partial) and rotates past refunded completed
  rows, so refund-then-repay and equal-amount partials diverge onto distinct
  keys; an un-refunded completed row keeps its key (double-charge protection
  holds). Dedup hits on refunded rows now 409, never stale success.
- chargeFailureStatus default is 503 (ambiguous), never 402; table test.
- Flaky TestBookingPayment_FullPayment_SplitsIntoDepositAndBalance fixed
  (ORDER BY payment_type).
- resolveChargeSource: orphaned card-on-file disabled via DeleteCardOnFile
  when SaveCardForUser fails (best-effort, redacted log); retry path preserved.

Square client:
- Dev builds HARD-FAIL (panic) on SQUARE_ENVIRONMENT=production without
  SQUARE_ALLOW_REAL_API=1; sandbox routes with a loud banner.
- Mock fault-injection FailAfterCommit (commit-then-5xx) exercises the exact
  lost-response same-key retry; SimulateCardTokenUsed; 45-char idempotency-key
  cap parity; SquareEnvironment/SquareLocationID shared env helpers used by
  the sweep (env contract no longer comment-only).
- listRefunds truncation now errors (money-sensitive reconcile retries
  instead of over-refunding); getCardsOnFile truncation loudly logged.

Webhooks + 2FA:
- square-environment header checked fail-closed (403) when configured env is
  production/sandbox; dispatch DB work bounded by 30s timeout contexts.
- 2FA codes HMAC-SHA256 pepper'd (TWO_FACTOR_PEPPER) with legacy-hash
  migration + upgrade-on-verify; disable-flow mint cooldown (1/min, 429) caps
  the brute-force loop; in-lockout records never LRU-evicted.

Repo hygiene:
- env-docs CI gate green again (FRONTEND_ORIGIN + SQUARE_ALLOW_REAL_API +
  TWO_FACTOR_PEPPER documented; Vite DEV built-in allowlisted).
- Dead square_deposits schema dropped; obsidian/README/legal-page drift fixed
  (consumeradvice.scot signposting, CORS allowlist, p11 R3/P13, T1).
- 2FA disable residual documented; P6 email/SMS delivery and P12 sandbox
  smoke test remain the pre-go-live gates.

Verification: go test -tags test,dev -count=1 -parallel 8 ./... (20/20 ok),
go build ./... + -tags dev, go vet clean, svelte-check 0 errors, env-docs
gate OK, live deposit-required flow re-verified end-to-end (deposit £11
charged, square_payment_id recorded).

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

@@ -322,7 +322,14 @@
 			if (twoFactorBlocksSavedCards && selectedPaymentMethod) {
 				selectedPaymentMethod = '';
 			}
-			await submitAndProceed();
+			// Create the booking only if one does not already exist. A retry after
+			// a failed deposit charge (or a lost response) must NOT re-create a
+			// booking — the existing confirmedBooking is the one to charge, and
+			// the backend enforces one-active-booking for deposit-required users,
+			// so a second submission would 409 and orphan an unpaid booking.
+			if (!confirmedBooking) {
+				await submitAndProceed();
+			}
 			if (!confirmedBooking) {
 				toast.error('Booking was not created. Please try again.');
 				return;
@@ -2158,7 +2165,7 @@
 
 		<!-- Step 4: Payment & Confirmation (final step) -->
 		{#if currentStep === finalStep}
-			{#if confirmedBooking}
+			{#if confirmedBooking && (depositPaid || !depositRequired || confirmedBooking.deposit_paid)}
 				{@const isRequested = confirmedBooking.notes && confirmedBooking.notes.length > 0}
 				{@const bookingDate = parseWallClockDate(confirmedBooking.start_time)}
 				{@const dateStr = bookingDate.toLocaleDateString('en-GB', {