Add a per-process availability flag (app.health) that /readyz
reports as 503 OFFLINE while offline, so the load balancer
drains the instance and in-flight requests finish untouched.
Operator surface is app.srepl.main/set-offline and
app.srepl.main/set-online; a restart comes back online.
Covered by backend-tests.health-test; HA docs updated.
AI-assisted-by: muse-spark-1.3-contributor
