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/handlers/model_handlers.py

@@ -90,6 +90,7 @@ class ModelPageView:
         settings_service: SettingsManager,
         server_i18n,
         logger: logging.Logger,
+        page_context_provider: Callable[[web.Request], Dict[str, Any]] | None = None,
     ) -> None:
         self._template_env = template_env
         self._template_name = template_name
@@ -97,6 +98,7 @@ class ModelPageView:
         self._settings = settings_service
         self._server_i18n = server_i18n
         self._logger = logger
+        self._page_context_provider = page_context_provider
 
     def _load_supporters(self) -> dict[str, Any]:
         """Load supporters data from JSON file."""
@@ -210,6 +212,16 @@ class ModelPageView:
                     self._logger.error("Error loading cache data: %s", cache_error)
                     template_context["is_initializing"] = True
 
+            if self._page_context_provider is not None:
+                try:
+                    extra_context = self._page_context_provider(request)
+                    if isinstance(extra_context, dict):
+                        template_context.update(extra_context)
+                except Exception as context_error:  # pragma: no cover - logging path
+                    self._logger.error(
+                        "Error building page context: %s", context_error
+                    )
+
             rendered = self._template_env.get_template(self._template_name).render(
                 **template_context
             )