Rewrite README.md from a basic scaffold into a complete service
description and specification document.
- Define naming scheme: Processor → Webhook, Webhook → Entrypoint
- Document all 8 data model entities with field tables and relations
- Describe separate database architecture (main app DB + per-webhook
event DBs)
- Document all API endpoints (current and planned)
- Detail middleware stack, authentication, security measures
- Include complete package layout with file descriptions
- Add request flow diagram
- Reorganize TODO into phased roadmap with issue links
- Set license to MIT
closes#3
