Report when the deployed policyengine.py version went live #4083

Problem

Nothing reports how long the API has served its current policyengine.py (.py) version. GET /v2/tax-benefit-models/by-country/{country} reports the version, but not since when. A deploy timestamp would not help: the API deploys a new Cloud Run revision several times a day while .py stays the same, so "last deployed" is nearly always today.

The app-v2 developer-tools Deployments page (PolicyEngine/policyengine-app-v2#1450) shows, for each core service, which .py, country model and data build it serves. It already shows "live since" for the household API (from its /versions deployed_at); the API has no equivalent.

Proposal

• At candidate deploy, read the .py version from uv.lock, and read DEPLOYED_POLICYENGINE_VERSION and POLICYENGINE_VERSION_LIVE_SINCE from the revision that serves 100% of traffic (steps.previous.outputs.revision). Keep its date when the .py version is unchanged; use the deploy time for a new version; leave it unknown when the serving revision has no recorded date. Pass both as Cloud Run environment variables.
• Add GET /deployment next to /health, returning the installed .py version, its live-since date and the Cloud Run revision (K_REVISION).

Done when

• A deploy that keeps the .py version keeps the date; a deploy that changes it (or a first deploy) sets a new date; a serving revision without a recorded date, or a failed lookup, leaves the date unknown rather than wrong, and never fails the deploy.
• GET /deployment returns the three fields; the date is null on a revision deployed before this change.