Better Timeout Logic; Added Idempotency Key to DB

[Howard-nolan]

Change Summary:

• Added request timeouts, readiness endpoint, and cache-aware handlers with idempotent shorten inserts; wrapped the server in http.TimeoutHandler and handled DB timeouts explicitly.
• Introduced idempotency key support (schema + insert logic) and documented readiness in OpenAPI; added ready route, updated README with test commands.
• Expanded tests: handler unit tests now cover timeouts/idempotency and redirect timeouts; integration test runs against in-memory DB.

How It Works:

• ShortenHandler reads the request ID as an idempotency key, uses ExecContext with an upsert-by-key, returns 408 on context cancel, and reuses the existing code on conflict.
• RedirectHandler checks cache, then QueryRowContext; returns 408 on context cancellation, 404 on miss.
• /readyz pings the DB with PingContext; server wraps the router with http.TimeoutHandler. Schema includes a unique idempotency_key on links.

Additional Notes (optional):

• Integration tests run with go test -tags=integration ./...; unit tests via go test ./....