Dockerize backend, frontend, and database into separate containers

- backend/Dockerfile: multi-stage Python build (builder + slim runtime)
- frontend/Dockerfile: multi-stage Node build + nginx:alpine serving
- frontend/nginx.conf: SPA routing + /api/ reverse proxy to backend
- docker-compose.yml: production compose with health checks and proper dependency ordering
- docker-compose.dev.yml: dev overrides with hot reload via volume mounts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -15,35 +15,51 @@ A fullstack SaaS web application built with FastAPI, React, and PostgreSQL.
 - User registration and login (JWT auth)
 - Protected dashboard route
 - `/api/users/me` — authenticated user info
-- Vite dev server proxies `/api` to FastAPI — no CORS issues in development
+- 3 separate Docker containers: `db` (PostgreSQL), `backend` (FastAPI), `frontend` (nginx)
+
+## Containers
+
+| Container | Image | Port | Description |
+|---|---|---|---|
+| `db` | postgres:16-alpine | 5432 | PostgreSQL database |
+| `backend` | custom (python:3.12-slim) | 8000 | FastAPI management API |
+| `frontend` | custom (nginx:alpine) | 80 | React UI served by nginx |
+
+The frontend nginx container proxies `/api/*` to the backend container internally — no CORS headers needed in production.
 
 ## Installation
 
 ### Prerequisites
 
-- Python 3.11+
-- Node.js 18+
-- PostgreSQL 16 (or Docker)
+- Docker + Docker Compose
 
-### Option A — Docker (recommended)
+### Production
 
 ```bash
 git clone <repo>
 cd destroying_sap
-cp .env.example backend/.env
-docker compose up --build
+cp .env.example backend/.env   # edit SECRET_KEY at minimum
+docker compose up --build -d
 ```
 
-- Frontend: http://localhost:5173
-- Backend / API docs: http://localhost:8000/docs
+- Frontend: http://localhost
+- API docs: http://localhost:8000/docs
 
-### Option B — Local
+### Development (hot reload)
+
+```bash
+docker compose -f docker-compose.yml -f docker-compose.dev.yml up --build
+```
+
+- Frontend (Vite): http://localhost:5173
+- Backend (uvicorn --reload): http://localhost:8000
+
+### Local (no Docker)
 
 **1. Start PostgreSQL**
 
 ```bash
 docker compose up db -d
-# or use a local PostgreSQL instance and update backend/.env
 ```
 
 **2. Backend**
@@ -52,7 +68,7 @@ docker compose up db -d
 cd backend
 python -m venv .venv && source .venv/bin/activate   # Windows: .venv\Scripts\activate
 pip install -e ".[dev]"
-cp ../.env.example .env   # edit DATABASE_URL and SECRET_KEY as needed
+cp ../.env.example .env
 alembic upgrade head
 uvicorn app.main:app --reload
 ```
@@ -60,9 +76,7 @@ uvicorn app.main:app --reload
 **3. Frontend**
 
 ```bash
-cd frontend
-npm install
-npm run dev
+cd frontend && npm install && npm run dev
 ```
 
 ## Environment Variables