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.

py/routes/base_model_routes.py

@@ -149,6 +149,7 @@ class BaseModelRoutes(ABC):
             settings_service=self._settings,
             server_i18n=self._server_i18n,
             logger=logger,
+            page_context_provider=self._get_page_context_provider(),
         )
         listing = ModelListingHandler(
             service=service,
@@ -250,6 +251,10 @@ class BaseModelRoutes(ABC):
         """Get expected model types string for error messages - to be overridden by subclasses."""
         return "any model type"
 
+    def _get_page_context_provider(self):
+        """Optional hook returning extra template context for the page view."""
+        return None
+
     def _find_model_file(self, files):
         """Find the appropriate model file from the files list - can be overridden by subclasses."""
         return next((file for file in files if file.get("type") in MODEL_WEIGHT_FILE_TYPES and file.get("primary") is True), None)