docs(self-hosting): document HOST/SUPERMEMORY_HOST binding and loopback security (#1441)

[Adityakk9031]

Summary

Closes #1441
Follow-up to #1148

Addresses interface binding and loopback security for self-hosted instances:

1. Core Configuration:
   • Documents HOST and SUPERMEMORY_HOST environment variables in apps/docs/self-hosting/configuration.mdx.
   • Explains default binding behavior (0.0.0.0 / all interfaces) and contrasts with loopback isolation (127.0.0.1).
2. Network Security Guidance:
   • Adds explicit warnings regarding LAN exposure when running supermemory-server on shared networks (coffee shops, offices, hotels).
   • Explains how operators can enforce loopback isolation using SUPERMEMORY_HOST=127.0.0.1 or HOST=127.0.0.1.
   • Documents non-root workarounds (reverse proxy, port mapping, firewall rules) for legacy binary versions.
3. Quickstart Integration:
   • Adds loopback security tips to apps/docs/self-hosting/quickstart.mdx.