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/apps/annotations/admin.py

@@ -0,0 +1,19 @@
+from django.contrib import admin
+
+from apps.annotations.models import Bookmark, Note
+
+
+@admin.register(Bookmark)
+class BookmarkAdmin(admin.ModelAdmin):
+    list_display = ("user", "book", "page", "created_at")
+    list_select_related = ("user", "book")
+    search_fields = ("user__email", "book__title", "location_text")
+    list_filter = ("created_at",)
+
+
+@admin.register(Note)
+class NoteAdmin(admin.ModelAdmin):
+    list_display = ("user", "book", "page", "created_at", "updated_at")
+    list_select_related = ("user", "book")
+    search_fields = ("user__email", "book__title", "content", "location_text")
+    list_filter = ("created_at",)