Each kind reports two distinct windows. open_period_* is the window currently being accumulated against (what the party is filling right now); next_due_period_* is the window the platform will generate a report for next, together with its filing next_due_deadline. After a report is generated for the open window, next_due_* advances while open_* stays put until the calendar rolls over.
Always returns 200. When the party exists but reporting was never enabled (or was disabled), enabled is false and kinds is empty — this lets callers tell “party not found” (404) apart from “party not reporting”.
