Post an on-drop recurrence cycle

Availability-gated. Every drop links an input. Send multipart/form-data with one new input file (field file, optional kind per_run or fixture, default per_run, up to 4 MB) to post a cycle with that file, or JSON attachment_ids to reuse 1 to 20 of the accepted template's attachments. The cycle also carries the contract's reused (fixture) attachments; a file already carried is not attached twice. The response returns upload_token and upload_url for more files, as after posting a bounty. Any unresolved open, in-progress, or delivered cycle blocks another drop, and drops are at least one hour apart.