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/config/urls.py

@@ -1,5 +1,10 @@
 from django.urls import path, include
+from drf_spectacular.views import SpectacularAPIView, SpectacularSwaggerView
 
 urlpatterns = [
+    path('api/v1/', include('api.v1.urls')),
     path('api/', include('contacts.urls')),
+    path('api/v1/schema/', SpectacularAPIView.as_view(), name='schema'),
+    path('api/v1/docs/', SpectacularSwaggerView.as_view(url_name='schema'), name='swagger-ui'),
+    path('api/v1/', include('core.auth_urls')),
 ]