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/urls.py

@@ -0,0 +1,9 @@
+from django.urls import path
+
+from . import views
+from .base import plugin_urlpatterns
+
+urlpatterns = [
+    path('plugins/', views.plugin_manifest, name='plugin-manifest'),
+    *plugin_urlpatterns(),
+]