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/graph/views.py

@@ -0,0 +1,15 @@
+from rest_framework.decorators import api_view
+from rest_framework.response import Response
+
+from .services import build_full_graph, build_network_map_graph
+
+
+@api_view(['GET'])
+def graph_data(request):
+    return Response(build_full_graph())
+
+
+@api_view(['GET'])
+def network_map_graph(request):
+    map_id = request.query_params.get('map_id')
+    return Response(build_network_map_graph(map_id))