Add order search and stop half-saved orders

You're done when

• GET /api/orders with no parameters returns all orders in the database, newest first, with the default page and size.
• Orders with the same creation time always come in the same order, on every page.
• The customer and status filters work alone and together.
• The total count covers every matching order, not only the current page.
• page and size return the right slice, and a page after the last one is empty, not an error.
• A bad page, size or status gets a 400.
• The response has exactly the fields the brief lists.
• When an order fails for lack of stock, no stock changes, and the next valid order still goes through.
• When an order fails for lack of stock, no order is saved.
• When an order names a product that does not exist, nothing is saved and no stock changes.

What it measures

Your environment