URL Shortener

# URL Shortener PRD

## Overview
Build a URL shortener service with a web interface and REST API.

## Requirements
- Shorten a URL: generate a random 6-char slug or accept a custom alias
- Redirect: visiting /abc123 redirects to the original URL (301 redirect)
- Validation: reject invalid URLs, reject reserved slugs (api, admin, health)
- Click tracking: count total clicks per link
- Analytics page for each link: total clicks, clicks over time (last 7 days chart), referrer breakdown
- Dashboard: list all shortened URLs with click counts, creation date, copy button
- QR code generation for each shortened URL (displayed inline)
- API endpoints:
  - POST /api/shorten { url, customAlias? } returns { shortUrl, slug }
  - GET /api/stats/:slug returns { clicks, createdAt, originalUrl }
- Rate limiting: max 10 URLs per minute per IP
- Expiration: optional TTL on links
- Copy-to-clipboard button for shortened URLs
- Responsive web UI

## Tech Stack
- Any full-stack framework
- In-memory or file-based storage (no external database required)

## Scoring Criteria
- **Functional (40%)**: Shorten, redirect, analytics, QR code, API all work
- **Quality (20%)**: Input validation, rate limiting, handles edge cases
- **Fidelity (25%)**: All features present, clean dashboard, charts work
- **Speed (15%)**: Time bonus