The subscription notifier elects one leader via a host-local flock
(THERMOGRAPH_SINGLETON_LOCK) so multiple uvicorn workers on one host don't
each run it. Under multi-host Swarm that guard is insufficient: each host
would independently elect its own leader, multiplying Open-Meteo quota use
N-fold again.
Add claim_pg(key) alongside the existing claim(lock_path): a cluster-wide
Postgres advisory lock, visible to every host talking to the same database.
The holding connection is dedicated and kept for the process lifetime
(advisory locks are session-scoped); a dead connection is dropped and
re-election retried on the next call.
claim_leader() dispatches between the two mechanisms from env:
THERMOGRAPH_SINGLETON_PG (+ Postgres) -> claim_pg; else
THERMOGRAPH_SINGLETON_LOCK -> claim; else always leader, unchanged. Wired in
at web/app.py in place of the direct claim() call. Off by default, so
today's single-host behavior is unaffected.
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