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/users/admin.py

@@ -0,0 +1,11 @@
+from django.contrib import admin
+from django.contrib.auth.admin import UserAdmin as BaseUserAdmin
+
+from apps.users.models import User
+
+
+@admin.register(User)
+class UserAdmin(BaseUserAdmin):
+    """Admin config for the custom User model."""
+    list_display = ("email", "username", "is_staff", "is_active", "date_joined")
+    search_fields = ("email", "username")

backend/apps/users/apps.py

@@ -0,0 +1,7 @@
+from django.apps import AppConfig
+
+
+class UsersConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "apps.users"
+    label = "users"

backend/apps/users/models.py

@@ -0,0 +1,13 @@
+from django.contrib.auth.models import AbstractUser
+
+
+class User(AbstractUser):
+    """Custom user model. Uses email as the unique identifier field."""
+
+    class Meta:
+        db_table = "users_user"
+        verbose_name = "User"
+        verbose_name_plural = "Users"
+
+    def __str__(self) -> str:
+        return self.email or self.username

backend/apps/users/urls.py

@@ -0,0 +1,8 @@
+from django.urls import include, path
+from rest_framework.routers import DefaultRouter
+from rest_framework_simplejwt.views import TokenObtainPairView, TokenRefreshView
+
+urlpatterns = [
+    path("token/", TokenObtainPairView.as_view(), name="token_obtain_pair"),
+    path("token/refresh/", TokenRefreshView.as_view(), name="token_refresh"),
+]