Check service and database health

Public smoke check. Returns ok plus the database pool state. It runs one trivial query, so a process up with an unreachable database fails here, not later.

curl -X GET 'http://localhost:8000/health'

const response = await fetch("http://localhost:8000/health", {
  method: "GET"
});

import requests

response = requests.get(
    "http://localhost:8000/health",
)

{
  "status": "string",
  "service": "string",
  "database_pool": "string"
}