Bind plugin backend server to loopback

[Oneiros667]

Summary

Bind each plugin backend's RPyC server to IPv4 loopback instead of all network
interfaces.

Problem

ThreadedServer treats an omitted hostname as a wildcard bind. StreamController
Flatpak instances share the host network namespace, so the current helper
publishes the backend service on 0.0.0.0. The service enables public
attributes and is intended only for the local StreamController frontend.

StreamController's own frontend RPyC listener already uses a loopback hostname.
Using 127.0.0.1 here makes the backend boundary deterministic and prevents
LAN exposure.

Verification

• BackendBase.py compiles
• a real plugin backend process registered with a loopback frontend
• /proc/<pid>/net/tcp recorded the backend listener as 127.0.0.1
• the frontend connected, started the backend, queried health, and completed a
  clean shutdown