feat(backend): gate Other Models behind opt-in management toggles

Other Models management is now opt-in: enable_other_models (default false)
plus the enabled_other_sub_types allow-list replace the unreleased additive
enabled_other_folders key.

- config._get_enabled_other_folder_keys() is the single scan gate; a new
  refresh_other_roots() rebuilds roots and preview roots on toggle.
- ModelScanner gains a _should_keep_cached_entry() hydration hook and
  on_library_changed(reconcile=...) so switching a sub_type off drops its
  entries (and hash/autov3 rows) at load time and switching it on rescans.
- OtherScanner filters location-derived entries accordingly.
- Other routes reject every other type while off (or a disabled sub_type) and
  expose an "other_disabled" page flag; download routing returns a disabled
  marker instead of guessing; the download manager refuses other-type
  downloads and default-path routing for switched-off sub_types.
- Doctor / init-status / refresh-all skip the other scanner while off; the
  scanner stays registered so staged pending-deletes still merge.
- Tests updated with explicit opt-in fixtures plus new gating coverage.

tests/services/test_recipe_scanner.py

@@ -2513,7 +2513,7 @@ async def test_on_library_changed_bumps_cache_version(tmp_path: Path, monkeypatc
     scanner = DummyScanner(str(tmp_path))
     assert scanner.cache_version == 0
 
-    async def _noop_initialize() -> None:
+    async def _noop_initialize(reconcile: bool = False) -> None:
         pass
 
     monkeypatch.setattr(scanner, "initialize_in_background", _noop_initialize)