feat: bookmarks and notes management

- Backend: Django REST Framework API with Bookmark and Note models
  - ViewSets with user-scoped querysets and select_related for N+1 prevention
  - Create/List/Detail/Update/Delete endpoints
  - Batch delete operations
  - Unique constraint on user+book+page for bookmarks
  - IsOwner permission class for object-level access control
  - Full serializer validation (page > 0, non-empty content, duplicate check)
  - 30+ pytest-django tests covering CRUD, auth, filtering, edge cases

- Frontend: React TypeScript components
  - AnnotationsContext with useReducer for state management
  - BookmarkList, NoteList, AddAnnotationForm, AnnotationsDashboard
  - Inline note editing with immediate save
  - Batch delete support
  - API client with JWT auto-refresh interceptors
  - Paginated query hook for infinite scroll support
  - Responsive CSS with loading/empty states

- Infrastructure: Django project with custom User model, JWT auth, CORS
  - PostgreSQL database models with proper FK and indexes
  - Django admin configuration for all models

backend/manage.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python
+"""Django's command-line utility for administrative tasks."""
+
+import os
+import sys
+
+
+def main():
+    """Run administrative tasks."""
+    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "config.django")
+    try:
+        from django.core.management import execute_from_command_line
+    except ImportError as exc:
+        raise ImportError(
+            "Couldn't import Django. Are you sure it's installed and "
+            "available on your PYTHONPATH environment variable? Did you "
+            "forget to activate a virtual environment?"
+        ) from exc
+    execute_from_command_line(sys.argv)
+
+
+if __name__ == "__main__":
+    main()