Add backoff to proxy Accept errors

[Luqueee]

Problem

In internal/proxy/proxy.go:30-41, every non-closed listener error is logged and immediately retried.

Persistent errors such as EMFILE or ENFILE can cause a tight CPU/logging loop, making an already degraded process less stable.

Acceptance criteria

• Add bounded exponential backoff for repeated Accept failures.
• Reset the backoff after a successful accept.
• Continue returning cleanly for net.ErrClosed.
• Add a deterministic test using a listener that repeatedly returns an error.