feat: set up monorepo scaffold with Yarn workspaces, Django API, React/Vite web, and Docker Compose

- Root package.json with Yarn workspaces (web, api)
- /web: React 19 + Vite 6 + @vitejs/plugin-react
  - index.html, src/main.jsx, src/App.jsx, vite.config.js
- /api: Django 5.x managed with uv
  - pyproject.toml with Django + psycopg2-binary
  - project/settings.py with PostgreSQL config from DATABASE_URL
  - project/urls.py, project/wsgi.py, manage.py
- docker-compose.yml with db (postgres:15), api, web services
  - Health check for db, live code volumes, dependency ordering
- Dockerfiles for both web (node:20-alpine) and api (python:3.12-slim)
- .gitignore updated for Python build artifacts and venv
- docs/implementation-issue-1-monorepo-scaffold.md

web/src/main.jsx

@@ -0,0 +1,12 @@
+import React from "react";
+import ReactDOM from "react-dom/client";
+import App from "./App";
+
+const rootElement = document.getElementById("root");
+if (!rootElement) throw new Error("Root element not found");
+
+ReactDOM.createRoot(rootElement).render(
+  <React.StrictMode>
+    <App />
+  </React.StrictMode>,
+);