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_other_scanner.py

@@ -173,6 +173,51 @@ class TestOtherScannerRoots:
         assert result["sub_type"] == "upscaler"
 
 
+class TestOtherScannerHydrationFilter:
+    """Persisted entries for roots that are no longer managed are dropped."""
+
+    def test_keeps_entries_under_enabled_roots(self, other_config):
+        scanner = _make_scanner()
+        assert (
+            scanner._should_keep_cached_entry(
+                {"file_path": f"{other_config['vae']}/model.safetensors"}
+            )
+            is True
+        )
+
+    def test_drops_entries_under_disabled_root(self, other_config, monkeypatch):
+        scanner = _make_scanner()
+        # Only vae stays managed; the upscaler root disappeared from the map.
+        monkeypatch.setattr(
+            config_module.config,
+            "other_root_subtypes",
+            {other_config["vae"]: "vae"},
+        )
+
+        assert (
+            scanner._should_keep_cached_entry(
+                {"file_path": f"{other_config['upscaler']}/model.safetensors"}
+            )
+            is False
+        )
+        assert (
+            scanner._should_keep_cached_entry(
+                {"file_path": f"{other_config['vae']}/model.safetensors"}
+            )
+            is True
+        )
+
+    def test_drops_everything_when_feature_off(self, monkeypatch):
+        monkeypatch.setattr(config_module.config, "other_root_subtypes", {})
+        scanner = _make_scanner()
+        assert (
+            scanner._should_keep_cached_entry(
+                {"file_path": "/models/vae/model.safetensors"}
+            )
+            is False
+        )
+
+
 class TestOtherScannerLazyHash:
     """Lazy hashing: pending by default, singleflight on-demand calculation."""