Register a dependency set to re-check later (one credit to create).

Requires an X-API-Key. Creating a watch spends one credit, taken only after the baseline is recorded, so a feed outage costs nothing. Send a lockfile (raw or as {"lockfile":"<contents>"}) or {"packages":[{"name","version"}]}: exactly pinned versions only, at most 2000. We store package identities and what we knew at that moment, never lockfile contents or code. Returns a watch_id and a watch_token shown once. A watch is re-checked when you read it with GET /v1/watch/{id}: nothing runs on a schedule and nothing is pushed to you, so poll it.