Add plugin platform with modular backend and frontend registry.

Split graph/import/meta into Django apps, add API v1 with OpenAPI and pytest, and introduce plugin registry with the tags reference plugin on both FE and BE.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/core/urls.py

@@ -0,0 +1,9 @@
+from django.urls import path
+
+from . import views
+
+urlpatterns = [
+    path('meta/choices/', views.meta_choices, name='meta-choices'),
+    path('relation-types/', views.relation_types, name='relation-types'),
+    path('network-map-choices/', views.network_map_choices, name='network-map-choices'),
+]