feat: add webhook server for standalone deployment

[wadefletch]

Summary

• Add src/server.ts — Bun HTTP server that receives GitHub workflow_run webhooks and runs the fixup engine
• Add Dockerfile for EKS deployment
• Uses GitHub App authentication (@octokit/auth-app) for org-wide access
• Webhook signature verification via X-Hub-Signature-256
• Async processing — returns 202 immediately, runs fixup in background

Changes

• src/server.ts: Webhook handler with health check endpoint
• Dockerfile: Alpine-based Bun image with git
• package.json: Added start script, @octokit/auth-app and @types/bun deps

The GitHub Action entry point (src/index.ts, action.yml, dist/index.js) is unchanged.