[Epic] TTL & record expiration correctness #2116

Record expiration and TTL: the eviction path, the accounting behind it, and what happens when an expiration value is wrong.

Scope

In: TTL/expiration configuration and validation, the eviction and cleanup path, whether expired records actually stop being returned, and the disk/memory accounting of records past their expiry.

Out: transaction-log and audit retention (harper#2115) — related in spirit, different mechanism and different config surface.

Why

Two of the members fail in ways that look nothing alike but share a cause: expiration values reach the storage layer without having been validated against what that layer accepts, and expired records are treated as gone by one layer while another still returns them. Both are the kind of defect that reads as a leak until you look closely.

Created during backlog triage. Child issues keep their own priorities and milestones.