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/api/v1/__init__.py

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+

backend/api/v1/urls.py

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+from django.urls import path, include
+from rest_framework.routers import DefaultRouter
+
+from contacts import views as contact_views
+
+router = DefaultRouter()
+router.register('contacts', contact_views.ContactViewSet)
+router.register('relations', contact_views.RelationViewSet)
+router.register('network-maps', contact_views.NetworkMapViewSet)
+
+urlpatterns = [
+    path('', include(router.urls)),
+    path(
+        'network-maps/<int:map_pk>/memberships/',
+        contact_views.NetworkMapMembershipViewSet.as_view({'get': 'list', 'post': 'create'}),
+        name='network-map-memberships-list',
+    ),
+    path(
+        'network-maps/<int:map_pk>/memberships/<int:pk>/',
+        contact_views.NetworkMapMembershipViewSet.as_view({
+            'get': 'retrieve',
+            'patch': 'partial_update',
+            'put': 'update',
+            'delete': 'destroy',
+        }),
+        name='network-map-memberships-detail',
+    ),
+    path('', include('core.urls')),
+    path('', include('graph.urls')),
+    path('', include('import_export.urls')),
+    path('', include('plugins.urls')),
+]