docs: client authentication, connect uid and Japa testing

[LeCoupa]

What

Extends the README with the three areas that were missing:

• Channel Authorization — explains that __transmit/subscribe / __transmit/unsubscribe are public by default, so ctx.auth.user is undefined unless those routes go through your auth middleware. Adds a transmit.registerRoutes snippet to protect them.
• Client (new section) — installing @adonisjs/transmit-client, creating the client, and using the beforeSubscribe hook to attach an Authorization: Bearer <token> header on every subscription request, plus subscribing and listening for messages.
• Events — adds a note clarifying that the connect / disconnect events expose the client uid, which can be mapped to the authenticated user.
• Testing (new section) — shows how to assert on broadcasts in Japa by collecting them through transmit.on('broadcast', ...) (which returns a stop function), and how to test that an unauthenticated subscription is rejected with a 401.
• Updates the Table of Contents (Client, Testing).

Why

The docs only covered server-side transmit.authorize, with no guidance on authenticating the client, on the lifecycle uid, or on testing broadcasts. These are common needs when securing private channels.

Notes

• beforeSubscribe receives a single Request argument, so headers are mutated via request.headers.set(...).
• The subscribe route is a POST, so the auth test posts to /__transmit/subscribe.

[thetutlage]

Thanks 👍🏽

[RomainLanz]

Could you also provide a PR for the documentation here https://github.com/adonisjs/v7-docs ?

[LeCoupa]

yes i will do it on monday :)