chore(coordinator): add API smoke script and request collection

Two ways to exercise every endpoint, both living in the repo rather than in a
personal Postman workspace:

- scripts/smoke.sh walks the full lifecycle and asserts each status, exiting
  non-zero on the first surprise, so it works in CI as well as by hand;
- api/requests.http drives the same calls from an editor's REST client, with
  later requests reusing ids captured from earlier responses. It doubles as
  API documentation for the worker author.

The script claims until it sees its own job's chunks instead of assuming an
empty queue: a shared development database usually holds pending tasks from
earlier runs, and it takes the attempt number from the claim response, since a
task requeued after an expired lease comes back with attempt 2 or 3.

Note for whoever extends the validation cases: Go matches JSON field names
case-insensitively, so "worker_ID" is accepted as "worker_id". Only a genuinely
unknown key trips DisallowUnknownFields.

coordinator/README.md

@@ -111,6 +111,18 @@ See `.env.example`; only `DATABASE_URL` is required.
 | GET    | `/jobs/{job_id}`              | Aggregate job progress                     |
 | GET    | `/health`                     | Liveness (unauthenticated)                 |
 
+## Poking the API
+
+Two ways, both checked in:
+
+```sh
+make smoke                    # every endpoint, asserted; non-zero exit on failure
+```
+
+`api/requests.http` runs the same calls one at a time from an editor with a REST
+client (VSCodium/VS Code "REST Client", JetBrains HTTP Client). Later requests
+reuse ids captured from earlier responses, so it doubles as API documentation.
+
 ## Status
 
 The queue works end to end: a job can be submitted, split into tasks, leased to