Build my first API endpoint

My project is <one sentence>, built with <stack> and hosted on <host>.
I need an endpoint: <METHOD> <path> that <does what>.

Build it with:
- Routing done this stack's normal way.
- Every input validated, with clear 400-style responses.
- The right status code for each outcome.
- One consistent error format I can copy to every future endpoint.
- Anything this host needs for the route to be reachable, such as
  rewrite rules.

Then show me how to call it with curl so I can test it on the live host.