Inventory Webhook Receiver

Problem/Feature Description

A retail company needs a Go HTTP service that receives inventory update webhooks from their warehouse management system. When the warehouse ships items, adjusts stock, or receives new inventory, it sends a webhook to this service. The service validates the webhook, updates an in-memory inventory map, and returns appropriate status codes.

The service needs these endpoints:

• POST /webhooks/inventory — Receive an inventory webhook. The request body contains: event_type (one of: "stock_received", "stock_shipped", "stock_adjusted"), sku, quantity (integer, positive for additions, negative for removals), warehouse_id, and timestamp. Validate that all required fields are present and quantity is non-zero. If the SKU doesn't exist, create it. If a shipment would bring stock below zero, reject with 409 Conflict.
• GET /inventory/:sku — Get current stock level for a SKU. Returns 404 if SKU not found.
• GET /inventory — List all SKUs and their current stock levels.
• GET /health — Health check endpoint, returns 200 OK.

Use Go's standard library net/http. The service will run in a Kubernetes cluster with Fluent Bit collecting stdout for Elasticsearch.

Output Specification

Produce:

• output/go.mod — Module file with dependencies
• output/main.go — Server setup and entry point
• output/handlers.go — HTTP handler functions
• output/middleware.go — Any middleware needed

Complete Go code, no placeholders or TODO comments.