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_graph_api.py

@@ -0,0 +1,22 @@
+import pytest
+
+
+@pytest.mark.django_db
+def test_graph_empty(api_client):
+    response = api_client.get('/api/v1/graph/')
+    assert response.status_code == 200
+    assert response.data == {'nodes': [], 'edges': []}
+
+
+@pytest.mark.django_db
+def test_graph_with_data(api_client, sample_contact, sample_relation):
+    response = api_client.get('/api/v1/graph/')
+    assert response.status_code == 200
+    assert len(response.data['nodes']) == 2
+    assert len(response.data['edges']) == 1
+
+
+@pytest.mark.django_db
+def test_legacy_api_path(api_client):
+    response = api_client.get('/api/graph/')
+    assert response.status_code == 200