Enroll leads in enrichment: enriched now, re-checked monthly (SPENDS TOKENS)

One lead per two businesses, paid once. Each business is enriched right away and then re-checked about once a month for as long as you keep it, with change events on what moved (GET /api/v1/leads?signal=..., or the signal.detected webhook). Enrolling is separate from enriching (POST /api/enrich, one lead per five, which never enrolls). Businesses a search finds are already enrolled for free. Businesses already enrolled, and businesses with no website to re-check, are skipped and cost nothing. A business whose site cannot be read is charged the enrichment rate for the attempt and is not enrolled. Runs as a job: poll GET /api/enrich/progress/{jobId}, which reports enrolled, readOnly and leadsCharged. Price it first with POST /api/v1/enrich/preview (enrollCost). Lead-spending limit: 30,000 businesses a minute on Growth, up to 150,000 on Agency.