warm_cities.py and indexnow.py are one-shot scripts run manually at deploy
today - nothing keeps the curated city set topped up, or pings IndexNow,
between deploys.
Add notifications/scheduler.py: an in-process APScheduler instance driving
two recurring jobs (city warming, daily; IndexNow --if-changed, every 6h by
default), gated by the same leader election as the notifier - only the
process that already won leadership starts it, so the jobs run in exactly
one place under multi-host Swarm rather than once per replica. Neither job
runs immediately on start: warm_cities already runs at deploy time, so an
immediate duplicate on every worker boot/restart would be wasted work.
Extract indexnow.submit_if_changed() from the CLI's --if-changed branch so
the scheduler and the command line share the exact same skip-when-unchanged
logic instead of two copies drifting apart. The CLI's plain (non-flag) path
is unchanged.
APScheduler over a Postgres-native queue: exactly one process ever runs
this, no fan-out/backpressure/dead-letter need, no new infrastructure.
