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/registry.py

@@ -0,0 +1,6 @@
+"""Register built-in plugins."""
+
+from plugins.base import register_plugin
+from plugins_tags.plugin import TagsPlugin
+
+register_plugin(TagsPlugin())