Record when each policyengine.py route was published and when latest last changed #960

Problem

The gateway's /versions reports which policyengine.py (.py) version each route serves and which one is latest, but not since when. The app-v2 developer-tools Deployments page (PolicyEngine/policyengine-app-v2#1450) shows, for each core service, the .py, model and data build it serves; it can show "live since" for the household API (its /versions has deployed_at) but not for the simulation API. GitHub's prod deployments are no substitute: they run several times a day and include worker rebuilds that route nothing new.

Proposal

Add two optional fields to the routing state that the routing publisher writes, and return them from GET /versions:

• published_at: for each .py version, when its route was first published (republishing the same version keeps the date).
• latest_since: when latest.policyengine last changed (a publish that advances latest, a forced latest, or a rollback).

Both are optional: states written before the change still read and validate, and the response shows {} / null until the next publish.

Done when

• build_next_routing_state and build_restored_routing_state set the fields as above; coerce_routing_state keeps valid values and drops malformed ones.
• VersionsResponse has published_at and latest_since, documented; the golden OpenAPI is updated.