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/conftest.py

@@ -0,0 +1,26 @@
+import pytest
+from contacts.models import Contact, Relation
+
+
+@pytest.fixture
+def api_client():
+    from rest_framework.test import APIClient
+    return APIClient()
+
+
+@pytest.fixture
+def sample_contact(db):
+    return Contact.objects.create(name='Иван Иванов', email='ivan@test.com')
+
+
+@pytest.fixture
+def two_contacts(db):
+    a = Contact.objects.create(name='Алиса')
+    b = Contact.objects.create(name='Боб')
+    return a, b
+
+
+@pytest.fixture
+def sample_relation(db, two_contacts):
+    a, b = two_contacts
+    return Relation.objects.create(source=a, target=b, relation_type='friend')