Fix review findings: BuyGiftCard concurrency lock, amount guards, NULL scan, mock dedup, docs

N1 (HIGH) — BuyGiftCard concurrent same-key retry could double-issue gift
cards (2× value for 1 charge). Added pg_advisory_lock on the idempotency key
(mirroring the tip pattern) acquired before the idempotency check, so
concurrent same-key retries serialize and only one executes gift-card
creation.

N2 — Amount-equality guards in both reuse branches (CreateTipPayment and
BuyGiftCard). A same-key retry with a different amount now returns 400
instead of silently mutating the pending record's books/VAT/refund caps.

N3 — test coverage:
- TestBuyGiftCard_RetryPending_ReattemptsCharge: pending record + same-key
  retry re-attempts, reuses the record (count=1), completes, and issues the
  gift card exactly once.
- TestCreateCheckoutHTTP_DeviceOptionsWireShape: httptest.Server asserts
  device_id is under checkout.device_options (not top-level). Extracted
  createCheckoutHTTPWithClient for injectable base URL.
- MockClient.CreatePayment now dedups on idempotency key (paymentByKey map),
  matching real Square behaviour.

N4 — Corrected the savepoint comments in handlers.go and giftcards.go: the
savepoint only exists in the test harness; in production db.Conn.Begin is a
plain tx and the status UPDATE runs on a separate pooled connection. Commit
is a harmless no-op in prod but required in tests.

Bonus bug fixed: CheckIdempotencyByKey scanned NULL booking_id/gift_card_id
(gift-card purchases) into plain string, failing with 'cannot scan NULL'.
Now uses sql.NullString.

Docs: Technical Manual.md:53 and Feature Catalog.md (2.1, 2.5) corrected —
no longer claim Web Payments SDK is live; new-card entry is documented as
pending P11, saved-card flow works via ccof tokens, dev mock rejects raw PANs.

backend/internal/square/square_http_client_test.go

@@ -2,7 +2,13 @@
 
 package square
 
-import "testing"
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
 
 func TestPaymentFromSquare_ElseBranch_SurfacesBrandWithoutCardID(t *testing.T) {
 	p := &sqPayment{
@@ -43,3 +49,54 @@ func TestPaymentFromSquare_NilCardDetails(t *testing.T) {
 		t.Errorf("expected amount 2500, got %d", result.Amount)
 	}
 }
+
+// TestCreateCheckoutHTTP_DeviceOptionsWireShape verifies the terminal checkout
+// request puts device_id inside checkout.device_options (Square's required
+// shape), not at the top level. A top-level device_id is rejected with 400 by
+// Square's real API.
+func TestCreateCheckoutHTTP_DeviceOptionsWireShape(t *testing.T) {
+	var captured map[string]any
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodPost {
+			t.Errorf("expected POST, got %s", r.Method)
+		}
+		if r.URL.Path != "/v2/terminals/checkouts" {
+			t.Errorf("expected /v2/terminals/checkouts, got %s", r.URL.Path)
+		}
+		if err := json.NewDecoder(r.Body).Decode(&captured); err != nil {
+			t.Errorf("failed to decode request body: %v", err)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"checkout":{"id":"chk_test","status":"PENDING","amount_money":{"amount":5000,"currency":"GBP"},"created_at":"2026-07-31T00:00:00Z","updated_at":"2026-07-31T00:00:00Z"}}`))
+	}))
+	defer srv.Close()
+
+	hc := &httpClient{baseURL: srv.URL, token: "test-token", http: srv.Client()}
+
+	_, err := createCheckoutHTTPWithClient(context.Background(), CreateCheckoutReq{
+		Amount:         5000,
+		Currency:       "GBP",
+		IdempotencyKey: "ik-1",
+		DeviceID:       "dvc_test",
+	}, hc)
+	if err != nil {
+		t.Fatalf("createCheckoutHTTP failed: %v", err)
+	}
+
+	checkout, ok := captured["checkout"].(map[string]any)
+	if !ok {
+		t.Fatalf("expected checkout object in body, got %v", captured)
+	}
+	// device_id must NOT be at the top level
+	if _, hasTopLevel := captured["device_id"]; hasTopLevel {
+		t.Errorf("device_id must not be top-level in terminal checkout request: %v", captured)
+	}
+	// device_id must live under checkout.device_options
+	devOpts, ok := checkout["device_options"].(map[string]any)
+	if !ok {
+		t.Fatalf("expected checkout.device_options in body, got %v", checkout)
+	}
+	if devOpts["device_id"] != "dvc_test" {
+		t.Errorf("expected checkout.device_options.device_id = dvc_test, got %v", devOpts)
+	}
+}