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/plugins/views.py

@@ -0,0 +1,19 @@
+from django.urls import path
+from rest_framework.decorators import api_view
+from rest_framework.response import Response
+
+from plugins.base import get_enabled_plugins
+
+
+@api_view(['GET'])
+def plugin_manifest(request):
+    """List enabled plugins and their metadata."""
+    return Response([
+        {
+            'id': p.id,
+            'version': p.version,
+            'min_core_version': p.min_core_version,
+            'permissions': p.permissions,
+        }
+        for p in get_enabled_plugins()
+    ])