Build a URL shortener service

• shorten(longUrl, { alias?, ttlMs? }) returns a short code; the URL is validated (http or https only)
• Codes are generated from an auto-increment id encoded in base62 (0-9a-zA-Z), so they are short and unique; a custom alias is used instead when given
• A custom alias that is already taken, or is invalid (length and character set), is rejected with a clear error; generated codes never collide with existing aliases
• resolve(code) returns the long URL and increments the click count; an expired or unknown code returns a not-found error, and the clock is injectable for tests
• stats(code) returns the long URL, creation time, expiry and click count
• Shortening the same long URL twice either returns the existing code or a new one, as a documented and tested policy

Only after every requirement works.

• Randomised, non-guessable codes with a collision retry loop
• Background or lazy purge of expired entries
• Per-user quotas and rate limiting