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/tests/test_contacts_api.py

@@ -0,0 +1,24 @@
+import pytest
+from django.urls import reverse
+
+
+@pytest.mark.django_db
+def test_contacts_list(api_client, sample_contact):
+    response = api_client.get('/api/v1/contacts/')
+    assert response.status_code == 200
+    assert response.data['count'] == 1
+    assert response.data['results'][0]['name'] == 'Иван Иванов'
+
+
+@pytest.mark.django_db
+def test_contact_create(api_client):
+    response = api_client.post('/api/v1/contacts/', {'name': 'Новый контакт'}, format='json')
+    assert response.status_code == 201
+    assert response.data['name'] == 'Новый контакт'
+
+
+@pytest.mark.django_db
+def test_relations_list(api_client, sample_relation):
+    response = api_client.get('/api/v1/relations/')
+    assert response.status_code == 200
+    assert response.data['count'] == 1