feature/backoffice (#6)

Co-authored-by: matst80 <mats.tornberg@gmail.com>
Reviewed-on: https://git.tornberg.me/mats/go-cart-actor/pulls/6
Co-authored-by: Mats Törnberg <mats@tornberg.me>
Co-committed-by: Mats Törnberg <mats@tornberg.me>

pkg/cart/mutation_remove_item.go

@@ -0,0 +1,45 @@
+package cart
+
+import (
+	"fmt"
+
+	messages "git.tornberg.me/go-cart-actor/pkg/messages"
+)
+
+// mutation_remove_item.go
+//
+// Registers the RemoveItem mutation.
+//
+// Behavior:
+//   - Removes the cart line whose local cart line Id == payload.Id
+//   - If no such line exists returns an error
+//   - Recalculates cart totals (WithTotals)
+//
+// Notes:
+//   - This removes only the line item; any deliveries referencing the removed
+//     item are NOT automatically adjusted (mirrors prior logic). If future
+//     semantics require pruning delivery.item_ids you can extend this handler.
+//   - If multiple lines somehow shared the same Id (should not happen), only
+//     the first match would be removed—data integrity relies on unique line Ids.
+
+func RemoveItem(g *CartGrain, m *messages.RemoveItem) error {
+	if m == nil {
+		return fmt.Errorf("RemoveItem: nil payload")
+	}
+	targetID := uint32(m.Id)
+
+	index := -1
+	for i, it := range g.Items {
+		if it.Id == targetID {
+			index = i
+			break
+		}
+	}
+	if index == -1 {
+		return fmt.Errorf("RemoveItem: item id %d not found", m.Id)
+	}
+
+	g.Items = append(g.Items[:index], g.Items[index+1:]...)
+	g.UpdateTotals()
+	return nil
+}