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/apps.py

@@ -0,0 +1,9 @@
+from django.apps import AppConfig
+
+
+class PluginsConfig(AppConfig):
+    default_auto_field = 'django.db.models.BigAutoField'
+    name = 'plugins'
+
+    def ready(self):
+        import plugins.registry  # noqa: F401