Fix P0/P1 review findings: truncation, raw-PAN API edge, refund lock, till pending-retry, idempotency keys

P0 — float truncation: applied math.Round to all remaining int64(x*100)
sites (till penceAmount, refund over-refund guard, GetAlreadyRefundedAmount,
payment summary conversions). A £1.14 till sale previously charged 113p.

P0 — raw PAN stopped at the API edge:
- Deleted CardNumber/CardExpMonth/CardExpYear/CardCVC from TillSaleRequest
  and CardNumber/Expiry/CVC from CreatePaymentMethodRequest. Both now accept
  card_token (Square nonce) and return 400 when absent. PAN+CVV no longer
  transit the application server (PCI-DSS SAQ-A scope).
- Deleted CreateCardOnFileRaw from the SquareClient interface and all
  implementations (MockClient, ProdClient, devProdClient).
- Added idempotency_key column to refunds table (UNIQUE).

P0 — RefundPayment hardened: advisory lock on payment ID (prevents two
concurrent refunds passing the over-refund guard), pending-refund-record-
then-Square pattern (scheduler reprocesses on failure), same-key dedup.

P1 — till sale pending-retry now re-attempts the Square charge instead of
returning the stale 'pending' status (gift card was already funded in the
committed tx — silent money loss otherwise). Sale row reused, not duplicated.

P1 — idempotency key caching in frontend: BuyGiftCard and
UserPaymentModal/BookingFlow now cache the key per amount+card, regenerated
on change and cleared on success — matches the tip-flow pattern so a
lost-response retry dedups instead of double-charging.

P1 — CreateTerminalPayment cash/giftcard INSERTs now persist idempotency_key.
Key is unique per payment (booking+type+amount would wrongly dedup two
legitimate identical payments, e.g. two £50 cash receipts).

P1 — gift-card codes no longer logged (spendable credential; value+recipient
only).

Tests: till pending-retry re-attempt, refund same-key dedup, mock CreatePayment
idempotency dedup, CreatePaymentMethod nonce happy path + raw-PAN rejection,
till online_square card_token required/valid.

backend/internal/square/square.go

@@ -2,10 +2,7 @@
 
 package square
 
-import (
-	"context"
-	"fmt"
-)
+import "context"
 
 var Client SquareClient
 
@@ -39,10 +36,6 @@ func (p *ProdClient) CreateCardOnFile(ctx context.Context, userID, cardToken str
 	return createCardOnFileHTTP(ctx, userID, cardToken)
 }
 
-func (p *ProdClient) CreateCardOnFileRaw(ctx context.Context, userID, cardNumber string, expMonth, expYear int, cvc string) (*CardOnFile, error) {
-	return nil, fmt.Errorf("square: raw card number input is not supported in production — use CreateCardOnFile with a card nonce")
-}
-
 func (p *ProdClient) GetCardsOnFile(ctx context.Context, userID string) ([]CardOnFile, error) {
 	return getCardsOnFileHTTP(ctx, userID)
 }