Create several atomizations for one article

Queues up to four channel formats for a single content item in one call. Requires the write:atomize scope.

Same gate order as POST /atomizations: body validation, then the plan gate (professional or higher), then the 20/hour per organization+user and 50/hour per organization limiters, then billing. types must be a non-empty array of at most 4 entries; unknown type values come back as a 400 with param: "input".

Billing is one charge for the whole batch, taken lazily just before the first type that actually dispatches — so a batch in which every type is already in flight costs nothing. overage_incurred is therefore a single flag for the call, not one per type.

Each type is processed sequentially: a row already at generating is reported back unchanged and skipped (no second dispatch), any other row is reset to generating, and new rows are inserted. Every dispatched type fires its own atomization/create background event, so the 201 means "queued", not "generated" — poll GET /atomizations/{id} for each returned id. There is no transaction across the loop: a mid-loop failure can leave some types queued and the ledger charged, but a retry is free within the same billing period.