feat: add readiness endpoint for provider health

[arniesaha]

Summary

• add a dedicated /ready endpoint alongside /health
• return safe provider diagnostics and readiness reasons
• document the liveness vs readiness distinction in deploy docs

Why

/health only proves the process is up. For mux, operators also need a readiness signal that tells them whether the configured providers can actually serve traffic.

Testing

• npm test -- --run tests/app.test.ts