The quota sensor previously relied only on Home Assistant's restore_state
for its display after a restart, while the stage/area data is restored
through the integration's own Store (#116). Make the quota consistent with
that pattern: the stage coordinator now persists the rate-limit snapshot
in its Store and reseeds sepush._rate_limit from it on async_load_cache().

On the #116 restart path (the stage poll is skipped) the SePush cache is
now primed from disk, so the quota sensor reads it without a blocking
refresh. The read-guard and restorable_attrs remain as a fallback for a
fresh or corrupt cache. The test mock's rate_limit() now mirrors the
in-memory _rate_limit cache like the real client.