Only include plugin URLs when app is in INSTALLED_APPS

Skip dynamic plugin URL inclusion for plugins that are on disk but not
in Django INSTALLED_APPS to avoid RuntimeError when loading models.
Plugin installer adds apps to INSTALLED_APPS on install; this prevents
breakage when that step was missed or reverted.

pluginHolder/urls.py

@@ -99,7 +99,13 @@ urlpatterns = [
 ]
 
 # Dynamically include each installed plugin's URLs so /plugins/<plugin_name>/settings/ etc. work
+# Only include plugins that are in INSTALLED_APPS so Django can load their models.
+from django.conf import settings
+_installed_apps = getattr(settings, 'INSTALLED_APPS', ())
+
 for _plugin_name, _path_parent in _get_installed_plugin_list():
+    if _plugin_name not in _installed_apps:
+        continue
     try:
         # If plugin is from a source path, ensure it is on sys.path so import works
         if _path_parent not in sys.path: