Write comprehensive README with architecture diagram and quickstart

[geminimir]

Summary

Project README with architecture overview, quickstart guide, configuration reference, and deployment instructions.

Changes

• Architecture diagram (Mermaid or image)
• Quickstart: clone, docker compose up, configure, receive webhooks
• Configuration reference: all env vars, per-chain settings, webhook config
• Deployment guide: Docker, Kubernetes manifests
• Contributing guidelines

Tests

• Quickstart steps work on a fresh clone

Acceptance Criteria

A new developer can understand and run the project from the README alone.