Change several tasks at once

Applies ONE change to up to 200 tasks — the grid’s bulk bar, without its delete. action picks the change and takes only its own argument: status with status (a live key, as for attivita_set_status); assign with assignee_user_id and/or assignee_team_id (null clears); due with due_at (null clears the deadline); list with list_id (null takes the tasks out of their grouping); priority with priority. Only OPEN tasks the member behind this credential may bulk-edit change: without attivita:manage, the ones they created, that are assigned to them, or that are assigned to a colleague they currently stand in for. A task left alone is not an error — the answer counts what changed and lists every other id with its reason: not_found (no such task, or not one they may see), closed (done or cancelled; reopen it with attivita_set_status), not_editable, unchanged (already in that status) or recurring_deadline (a recurring task keeps the deadline it repeats from). Completing a recurring task creates its next occurrence, and people are told as for a single change. Idempotent: the same call twice changes nothing more.