Freshness and storage transparency signals.

Freshness block

GET /api/scrape-cache exposes:

• freshness.oldest_submission_utc
• freshness.newest_submission_utc
• freshness.total_submissions
• freshness.total_listings
• freshness.listings_with_source_observed_utc
• freshness.listings_with_source_business_url

Storage block

• mode and store name
• persisted entry count
• truncation state
• error state when reads/writes fail