homepage.json lives on the appdata volume today, written by whichever
process's notifier last refreshed it. Under Swarm each web replica has its
own disk, so a file only one replica's notifier ever writes leaves every
other replica reading a stale or missing feed indefinitely.
On Postgres, refresh()/load() go through store.py's existing derived-payload
table instead (a fixed sentinel key, since the feed isn't cell-scoped) - a
single upsert is already atomic, and every replica reads the same row.
store.IS_POSTGRES is False without THERMOGRAPH_DATABASE_URL, so dev/tests
keep the plain file path entirely unchanged; no existing test needed to
change.
Verified live against real Postgres: refresh() persists into the shared
derived table and load() reads the exact same feed back.
22019bc632