OpenAlex API now meters list queries; single-work lookups are free

OpenAlex responses now carry x-ratelimit-cost-usd headers. Without an api_key, all clients on one IP share a $0.10/day budget; list/filter queries (/works?filter=...) cost $0.001 each, while single-entity lookups (/works/doi:10.xxx) cost $0 but count against a ~1000 calls/day limit. The mailto param does not raise the limit. For enrichment of a few hundred DOIs, loop over singleton lookups and cache to disk; reserve filter queries (e.g. cites:W..., publication_year:>Y) for the few places you need them. Also note counts_by_year only covers about the last ten years, so per-year citation windows before ~2016 are truncated.