Kamal 2 w/ embedded NATS

[prog-supdex]

What is the purpose of this pull request?

Add two single‑server deployment options via Kamal 2:

1. HTTP Broadcast (SQLite + AnyCable HTTP adapter, no Redis/PostgreSQL)
2. Embedded NATS (single anycable‑go instance runs both WebSocket server and embedded NATS; Rails/RPC connect via NATS)
3. Cluster-Mode Embedded NATS (two anycable‑go instances form an embedded NATS cluster across separate hosts via WS_NATS_CLUSTER_HOST and WS_NATS_HOST)

All options allow deploying the stack on one or more hosts without external dependencies.

Changes overview

• config/deploy.yml
  • Default single-server template using HTTP broadcast with SQLite.
• config/deploy_single_server_nats.yml
  • Single-server template with embedded NATS (ports 4242/4242).
• config/deploy_cluster_nats.yml
  • Cluster-mode template:
    • Seed node on WS_NATS_CLUSTER_HOST with client port 4242 and cluster port 4243
    • Second node on WS_NATS_HOST with client port 4322 and cluster port 4323, routing to the seed
• README.md
  • Added ENV table
  • Added sections for single-server and cluster-mode Embedded NATS.

Checklist

[ ] I've added a Changelog entry 
[ ] I've added tests for this change (N/A for deployment scripts)
[x] I've described this change in documentation
[ ] I've fixed all review comments