feat: add product bundles

[rgomezcasas]

Product Bundles

Goal

Allow customers to buy a predefined group of products at a discounted price.

Requirements

• A bundle contains at least 2 active products.
• Each bundle has its own name and a discounted price lower than the sum of its products.
• Customers cannot modify bundle contents.
• Adding a bundle to the cart adds a single line item.
• Bundles are shown in the catalog alongside individual products.
• Seed the database with a few default bundles.

Out of scope

• Dynamic or customer-configurable bundles.
• Per-customer or time-limited bundle discounts.