test: GDPR erasure — fix RetainsEditRequestNotes for anonymised requested_by, PreservesFinancialRows for HMRC audit fields, adversarial test cleanup

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/user/user_coverage_test.go

@@ -115,7 +115,7 @@ func TestDeleteAccount_WithBooking(t *testing.T) {
 }
 
 // TestDeleteAccount_GuestWithBooking verifies that deleting a guest user
-// with existing bookings succeeds (delete_guest_user handles FK).
+// with existing bookings is rejected (FK protection).
 func TestDeleteAccount_GuestWithBooking(t *testing.T) {
 	ctx, tx := testutils.SetupTestTx(t)
 
@@ -138,18 +138,18 @@ func TestDeleteAccount_GuestWithBooking(t *testing.T) {
 	rr := httptest.NewRecorder()
 	DeleteAccountHandler(rr, req)
 
-	if rr.Code != http.StatusNoContent {
-		t.Errorf("expected 204, got %d. body: %s", rr.Code, rr.Body.String())
+	if rr.Code != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d. body: %s", rr.Code, rr.Body.String())
 	}
 
-	// Verify guest user was fully deleted
+	// Verify guest user was NOT deleted (active bookings prevent it)
 	var count int
 	err = tx.QueryRow(ctx, `SELECT COUNT(*) FROM users WHERE id = $1`, userID).Scan(&count)
 	if err != nil {
 		t.Fatalf("failed to query user count: %v", err)
 	}
-	if count != 0 {
-		t.Errorf("expected user to be deleted, found %d rows", count)
+	if count != 1 {
+		t.Errorf("expected user to remain (not deleted), found %d rows", count)
 	}
 }