--system-site-packages only includes system-wide packages (/usr/lib/…);
it does NOT include user-installed packages (~/.local/lib/…).
When aisbf is installed with pip --user (or auto-detected user mode with
--break-system-packages), import aisbf fails inside the venv and the server
cannot start.

Add _link_user_site_packages() to aisbf.sh:
- Determines the user site-packages path via python3 -m site --user-site
- Writes a .pth file into the venv's site-packages directory pointing there
- Called on venv creation AND on every subsequent run (refresh after upgrades)

Also sync aisbf/aisbf.sh (the package-bundled bootstrap copy) to match.