Build-time HostSettings fields (multi_tenant, i18n_*, tenant_header) not marked requires_restart #499

Tier 1: correctness / UX

Problem

framework/hosting/simple_module_hosting/host_settings.py:75 declares multi_tenant: bool = False without the restart marker (_RESTART_*) that other build-time fields carry. create_app consumes multi_tenant (and i18n_*, tenant_header) during Phases 1 and 8, before the lifespan, so an admin-UI edit has no effect until a restart. The UI never says so.

CLAUDE.md already describes this exact trap ("this is why i18n_* and multi_tenant edits in the admin UI used to do nothing"). The restart flag is the missing half.

Fix

• Mark every field read in create_app as requires_restart.
• Add a test that walks the fields create_app reads and asserts each one carries the marker, so new build-time fields can't miss it.