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/routes/test_download_routing_handlers.py

@@ -5,6 +5,22 @@ import json
 import pytest
 
 from py.routes.handlers.download_routing_handlers import DownloadRoutingHandler
+from py.services.settings_manager import get_settings_manager
+
+
+@pytest.fixture(autouse=True)
+def enable_other_models():
+    """Other Models is opt-in; enable every sub_type for the routing tests."""
+    manager = get_settings_manager()
+    manager.settings["enable_other_models"] = True
+    manager.settings["enabled_other_sub_types"] = [
+        "vae",
+        "upscaler",
+        "text_encoder",
+        "clip_vision",
+        "controlnet",
+    ]
+    yield
 
 
 class FakeRequest:
@@ -149,3 +165,32 @@ async def test_other_invalid_selected_file_type_rejected():
         FakeRequest({"model_type": "VAE", "selected_file_type": 123})
     )
     assert response.status == 400
+
+
+@pytest.mark.asyncio
+async def test_other_routing_disabled_when_feature_off():
+    get_settings_manager().settings["enable_other_models"] = False
+
+    handler = DownloadRoutingHandler()
+    response = await handler.get_download_routing(
+        FakeRequest({"model_type": "VAE", "file_types": ["Model"]})
+    )
+    payload = json.loads(response.text)
+    assert payload["sub_type"] is None
+    assert payload["disabled"] is True
+    assert payload["reason"] == "other_models_disabled"
+
+
+@pytest.mark.asyncio
+async def test_other_routing_disabled_for_switched_off_sub_type():
+    get_settings_manager().settings["enabled_other_sub_types"] = ["vae"]
+
+    handler = DownloadRoutingHandler()
+    response = await handler.get_download_routing(
+        FakeRequest({"model_type": "Upscaler", "file_types": ["Model"]})
+    )
+    payload = json.loads(response.text)
+    assert payload["sub_type"] is None
+    assert payload["disabled"] is True
+    assert payload["reason"] == "other_sub_type_disabled"
+    assert payload["requested_sub_type"] == "upscaler"