Fix payment review round 3: saved-card idempotency, stale-pending sweep, webhook fail-closed

R1/R4: saved_card branch in CreateTerminalPayment now mirrors CreateTipPayment
- advisory lock (crussell:payment:<bookingID>) serializes concurrent double-clicks
- deterministic key bookingID-sc-type-amount-cardID (<=45 chars) so a lost-response
  retry derives the same key and dedups instead of double-charging
- idempotency switch inside the lock: completed -> dedup, pending -> reuse with
  pence amount-guard, failed -> clean 409
- success response includes card_brand/card_last4 (frontend already reads them)

R2: add 'failed' case to all four retry switches (tip, booking, gift card, till)
- a swept/definitively-rejected record returns 409 instead of 500-ing on the
  idempotency_key UNIQUE constraint

R3: extend SweepStalePendingPayments to till_sales card rows
- sweeps pending till_sales (online_square/in_person_card) past Square's ~24h
  key retention, closing the double-charge window for till sales
- swept rows logged with the same CRITICAL manual-reconciliation marker as the
  refund sweep

Webhook fail-closed: reject 503 when SQUARE_WEBHOOK_SIGNATURE_KEY unset, 403 on
bad signature (was: skip verification in dev)

Refund status resolution: refunds now resolve by Square status
(COMPLETED/PENDING/FAILED/REJECTED) instead of assuming completed; real error
codes (REFUND_AMOUNT_INVALID, PAYMENT_NOT_REFUNDABLE, REFUND_ALREADY_PENDING)
added to the definitive/processed classification

HTTP client: CreateCard key truncated to <=45 chars, device_options always sent
(env SQUARE_TERMINAL_DEVICE_ID fallback), processing_fee reads amount_money,
ListCards cursor loop, refund keys hashed to <=45 chars

Other fixes: payment/till/gift-card advisory-lock + FOR UPDATE asymmetries,
GetPaymentByID NULL scans, loyalty redemption lock, card upsert on conflict,
mock ccof: prefix parity, IsValidSquareCheckoutID for real Square IDs,
isAdminRequest defense-in-depth on all 6 admin payment handlers, webhook
signature docs, M8/L5 debug markers removed

Docs: README/FC/TM/Overview updated (22 jobs, 20 CRITICAL sites, 23-section
GDPR export, sweep jobs, webhook fail-closed); P11 plan marks remaining items
(sandbox smoke test, M-8 customer_id, saved-card key dedup trade-off) as
deferred with rationale; gap backlog pruned of completed items

backend/internal/validators/validators_test.go

@@ -76,3 +76,18 @@ func TestValidate_ValidStruct(t *testing.T) {
 	err := Validate.Struct(validStruct{Name: "hello"})
 	assert.NoError(t, err)
 }
+
+func TestIsValidSquareCheckoutID_AcceptsRealSquareID(t *testing.T) {
+	t.Parallel()
+	// Real Square checkout IDs are opaque UUIDs, not local 12-hex DB IDs.
+	assert.True(t, IsValidSquareCheckoutID("08YceKh7B3ZqO"))
+	assert.True(t, IsValidSquareCheckoutID("a1b2c3d4e5f6"))
+}
+
+func TestIsValidSquareCheckoutID_RejectsInvalid(t *testing.T) {
+	t.Parallel()
+	assert.False(t, IsValidSquareCheckoutID(""))
+	assert.False(t, IsValidSquareCheckoutID("../etc/passwd"))
+	assert.False(t, IsValidSquareCheckoutID("has spaces"))
+	assert.False(t, IsValidSquareCheckoutID("abc")) // too short
+}