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/services/other_scanner.py

@@ -454,6 +454,16 @@ class OtherScanner(ModelScanner):
             entry["sub_type"] = sub_type
         return entry
 
+    def _should_keep_cached_entry(self, entry: Dict[str, Any]) -> bool:
+        """Drop persisted entries whose folder is no longer a managed root.
+
+        sub_type is location-derived and config only maps enabled roots, so a
+        file under a disabled sub_type - or under any other root while the
+        feature is off - resolves to None here and is filtered out while the
+        persisted cache is hydrated.
+        """
+        return self.resolve_sub_type_for_path(entry.get("file_path")) is not None
+
     def get_model_roots(self) -> List[str]:
         """Get other-model root directories"""
         roots: List[str] = []