Keyword ideas from seed keywords

Description

Expands up to 200 seed keywords into related keyword ideas, each with search volume, CPC, competition and keyword difficulty (difficulty is at keyword_properties.keyword_difficulty in each row; volume and CPC at keyword_info). Ideas come from category-level expansion, so sorting by search volume surfaces unrelated head terms; the default order is relevance, keep it for topical work and filter instead. Cost scales with rows returned, so keep `limit` at the default 100 unless the task genuinely needs more (hard cap 1000).