Compat shims carry no removal version; add an enforced REMOVE_IN and retire them in a batch #515

Tier 3: deprecations

Shims with no end date

• host/routes_legacy.py:15: "Scheduled for removal one minor release after the move", with no version named. Added 2026-09-10. The redirects are cacheable 301s.
• simple_module_hosting/settings.py:1 calls Settings a "back-compat shim", yet it is the main type in about 35 files. Either it isn't a shim, or the migration was never finished.
• _registrations.py:66-78 keeps the one-argument register_event_handlers form. No module in the repo uses it any more.
• props.lazy() is deprecated in simple_module_inertia.
• Public routes have two mechanisms: the legacy AuthProvider.get_public_paths, which maintenance.py:106 still consults, and PublicRouteRegistry.

Fix

• Add a deprecated(remove_in="0.1.0") helper and a test that fails once the package version reaches remove_in. That forces a decision instead of letting shims live forever.
• Decide on the Settings naming: rename, or drop the "shim" wording.
• Move maintenance.py to PublicRouteRegistry and deprecate get_public_paths.