Set survey block order

Replaces the order of a survey's blocks. The body lists every current block id exactly once, in the order you want; block content is untouched.

Idempotent: the same order always yields the same survey, and an order that already matches performs no write at all, so updatedAt does not move and other callers' preconditions stay valid. A stale expectedUpdatedAt is still rejected with 409 on that path — the precondition is evaluated before the method, so a no-op outcome never reports success to a stale caller. Because the list must be a permutation, a dropped or duplicated block is rejected rather than silently applied — the failure mode that resending the whole blocks array cannot catch.

Pass expectedUpdatedAt (the updatedAt from your last read) to make the write conditional; see the 409 response. Session cookie or x-api-key.