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/books/serializers.py

@@ -0,0 +1,28 @@
+from rest_framework import serializers
+
+from apps.books.models import Book
+
+
+class BookSerializer(serializers.ModelSerializer):
+    """Serialize Book data."""
+
+    class Meta:
+        model = Book
+        fields = [
+            "id",
+            "title",
+            "author",
+            "total_pages",
+            "cover_image",
+            "created_at",
+            "updated_at",
+        ]
+        read_only_fields = ["id", "created_at", "updated_at"]
+
+
+class BookListSerializer(serializers.ModelSerializer):
+    """Lightweight serializer for list views (excludes heavy fields)."""
+
+    class Meta:
+        model = Book
+        fields = ["id", "title", "author", "total_pages", "cover_image"]