Cost-evaluation sweep (#215): re-classify each tenant that has pending index builds against its monthly cost cap and set/clear tenant.cost_capped_until.

This is the ONLY place tenant cost is classified for dispatch fairness; the drain just excludes capped tenants by the marker. Meant to run on its OWN (slow) cadence, decoupled from the hot dispatch drain -- a sibling of the reindex-catchup tick. A strict no-op when the monthly cost caps are dark (every tenant evaluates under cap, so no marker is set), so it is safe to schedule unconditionally. Takes no limit: it evaluates the whole (finite) set of tenants with pending builds each run so no over-cap tenant is left unflagged.