Translate Pending Kol Fields

Translate up to limit pending KOLs into zh-CN.

Returns a small summary: scanned (rows examined), translated (rows that got at least one new zh field), bypassed_cjk (rows whose source was already CJK — copied without an LLM call), llm_calls (gateway requests made), model (provider/model string used), error (set when the LLM call failed and the remaining batches were skipped; translated reflects partial progress before the failure).