Health tab ranks provider checks; backfill dry runs report true counts

Added

• Admin status page gains a Health tab — sources are ranked by last successful check timestamp (not release recency), with an amber/red alert banner when multiple orgs' checks have stalled; a frozen timestamp distinguishes a provider outage from an ordinary quiet period.
• Operator staleness digest now leads with AI provider quota alerts — when a provider cuts off access the digest reports it first: which provider, its stated restore time, and the verbatim message, so a shutoff surfaces in hours rather than days.

Fixed

• Backfill dry runs now report honest counts — inserted is null instead of a fabricated zero, and a notStored field shows exactly how many extracted URLs aren't yet in the index.