feat: add express-postgres N+1 example with slow/fast endpoints

[TarekHassan1]

What this changes

Adds a runnable examples/express-postgres/ directory that demonstrates the N+1 detection library against a real Express + PostgreSQL stack. Includes docker-compose.yml, a seed script, two endpoints (/orders/slow with the N+1 bug and /orders/fast with the fix), and a README.

Why this way

The existing examples/demo.mjs uses a fake client. This gives users a real, copy-paste-runnable example with an actual database so they can see the warning fire live against real queries.

How it was verified

• docker compose up -d → node seed.mjs → node server.mjs ran successfully locally
• curl localhost:3000/orders/slow triggers the expected N+1 warning (10× SELECT * FROM order_items WHERE order_id = ?)
• curl localhost:3000/orders/fast returns clean with no warning