Document self-hosting the controller

[dkontango]

Summary

The README explains the client side well but doesn't cover how to run your own enrollment controller. Anyone who wants to use schmutz-join needs a controller behind the /api/register and /api/enroll endpoints.

Proposal

Add a docs/SELF-HOSTING.md that covers:

• What the controller does (registration, enrollment proxy, config API)
• Prerequisites (OpenZiti controller, identity with admin access)
• Minimal config (env vars, listen address, Ziti connection)
• Caddy/nginx example for the public join endpoint
• How the /install → /download → /api/register → /api/enroll flow works
• How to set up the public/ catalog (OS versions for validation)

Notes

Keep it high-level — the controller is a separate private component. This doc explains the interface, not the implementation.