Build an Inventory Management API

Problem/Feature Description

A small warehouse company needs a REST API to track its inventory. Build it using Go with net/http (standard library).

Resources:

• Product: { id, sku, name, quantity, location }

Endpoints:

• GET /api/products -- list all products
• GET /api/products/{id} -- get a single product by ID
• POST /api/products -- create a product (sku required and must be unique, name required, quantity >= 0)
• PUT /api/products/{id} -- update a product
• DELETE /api/products/{id} -- delete a product
• POST /api/products/{id}/adjust -- adjust stock quantity (body: { "delta": 5 } or { "delta": -3 }). Quantity must not go below zero.

Use in-memory storage (no real database needed). Seed with 3 sample products.

Output Specification

Produce:

• main.go -- server entry point and route registration
• errors.go -- error types
• handlers.go -- route handlers
• models.go -- data types and in-memory store
• go.mod -- module definition

You may create additional files as needed for a well-structured codebase.