Add Railway deployment guide

[irinanazarova]

Summary

Adds a deployment guide for Railway, covering the full path from Solid Cable to AnyCable-Go in production.

Based on real migrations of two Rails apps (Fizzy and Solaris) to AnyCable on Railway, with every pitfall we hit documented inline.

What's covered

• AnyCable-Go service — Docker image setup, env vars, via dashboard and Railway CLI
• Rails configuration — gem, cable.yml, anycable.yml, initializer (secrets + Turbo verifier key in one file), production.rb
• Authentication — JWT meta tag for cross-domain, connection.rb guidance
• Client-side JS — importmap-rails (vendoring + alias pin + turbo import patching) and jsbundling-rails (just npm install)
• CSP — URI.parse-based origin extraction
• Development mode — keep Solid Cable for dev, AnyCable for prod
• Presence — complete Hotwire example with <turbo-cable-presence-source>
• Scaling — memory vs NATS broker
• Verification — browser DevTools checklist, broadcast test, Railway CLI logs
• Troubleshooting — integrated at each failure point

Pitfalls documented from real deployments

• anycable/anycable-go:latest may be too old for presence (pin to 1.6)
• @anycable/turbo-stream vendored file needs patching for importmap-rails (named imports vs Turbo namespace — this should be fixed in the npm package)
• signed_stream_verifier_key must be in after_initialize
• HTTP broadcast port is 8080 (not 8090) when ANYCABLE_SECRET is set
• ANYCABLE_URL (public, browser-facing) vs ANYCABLE_HTTP_BROADCAST_URL (private, *.railway.internal)
• CSP blocks WebSocket if connect-src doesn't include the AnyCable-Go origin
• System tests break silently if vendored JS imports fail

Note on @anycable/turbo-stream npm package

The package imports { connectStreamSource } from @hotwired/turbo as a named export. This works with jsbundling (real npm package) but breaks with importmap-rails (turbo.min.js from turbo-rails nests it under Turbo). This causes a silent SyntaxError that kills all JS on the page. The guide documents a manual patch, but the package itself should be fixed to handle both:

import * as TurboModule from "@hotwired/turbo"
const { connectStreamSource, disconnectStreamSource } =
  TurboModule.connectStreamSource ? TurboModule : TurboModule.Turbo

[palkan]

The only relevant bits are how to use railway cli to deploy anycable-go and CSP configuration (though it also must be a part of the general Rails guide). Everything else is unrelated to Railway deployment (and full of incorrect instructions, e.g., initializers, import maps, NATS, etc.).