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_settings_manager.py

@@ -1222,7 +1222,33 @@ def test_default_other_roots_stay_empty_without_other_folders(manager):
     assert manager.get("default_other_roots") == {}
 
 
+def test_other_models_disabled_by_default(manager):
+    assert manager.is_other_models_enabled() is False
+    assert manager.get_enabled_other_sub_types() == []
+    assert manager.is_other_sub_type_enabled("vae") is False
+
+
+def test_auto_set_default_other_roots_skipped_when_feature_off(manager):
+    manager.settings["enable_other_models"] = False
+    manager.settings["default_other_roots"] = {}
+    manager.settings["folder_paths"] = {"vae": ["/vae"]}
+
+    manager._auto_set_default_roots()
+
+    assert manager.get("default_other_roots") == {}
+
+
+def test_set_enabled_other_sub_types_normalizes(manager):
+    manager.settings["enable_other_models"] = True
+    manager.set("enabled_other_sub_types", ["controlnet", "vae", "nope", "vae", 42])
+
+    assert manager.get("enabled_other_sub_types") == ["vae", "controlnet"]
+    assert manager.is_other_sub_type_enabled("vae") is True
+    assert manager.is_other_sub_type_enabled("upscaler") is False
+
+
 def test_auto_set_default_other_roots(manager):
+    manager.settings["enable_other_models"] = True
     manager.settings["default_other_roots"] = {}
     manager.settings["folder_paths"] = {
         "vae": ["/vae"],
@@ -1243,6 +1269,7 @@ def test_auto_set_default_other_roots(manager):
 
 def test_auto_set_default_other_roots_text_encoder_dual_key_union(manager):
     """text_encoder candidates merge text_encoders and the legacy clip key."""
+    manager.settings["enable_other_models"] = True
     manager.settings["default_other_roots"] = {}
     manager.settings["folder_paths"] = {
         "clip": ["/legacy-clip"],
@@ -1261,6 +1288,7 @@ def test_auto_set_default_other_roots_text_encoder_dual_key_union(manager):
 
 
 def test_auto_set_default_other_roots_repairs_stale(manager):
+    manager.settings["enable_other_models"] = True
     manager.settings["default_other_roots"] = {"vae": "/stale-vae"}
     manager.settings["folder_paths"] = {"vae": ["/vae"]}
 
@@ -1270,6 +1298,7 @@ def test_auto_set_default_other_roots_repairs_stale(manager):
 
 
 def test_auto_set_default_other_roots_uses_extra_folder_paths(manager):
+    manager.settings["enable_other_models"] = True
     manager.settings["default_other_roots"] = {}
     manager.settings["folder_paths"] = {"vae": []}
     manager.settings["extra_folder_paths"] = {"vae": ["/extra-vae"]}