Create up to 100 tasks in one chunk

Ticket b4c525fa: { items: [...] } (max 100/req, named constant V0_BATCH_MAX_ITEMS) runs the single-create validation + quota + plan checks PER ITEM and returns per-item results [{ id } | { error: { code, message } }] in request order — one bad item never sinks the batch and the ordered array is the idMap the links phase needs. One multi-row insert per chunk; quota consumes per created item; quota-hit items fail per-item with 402 TASK_LIMIT. Runner marks NOT-RUNNABLE locally.