Coordinator serves the last-good snapshot within a staleness threshold

WHY: any single typed fetch failure was a failed pass, so the whole
board flipped unavailable on one dropped poll and stayed dark until the
API answered again — even though HA's coordinator machinery keeps the
last good data on a failed pass and the panel could safely keep
rendering it for a while.

The coordinator now caches every successful snapshot with its
time.monotonic stamp; while the elapsed time since the last success is
within the configured staleness threshold (CONF_SNAPSHOT_STALENESS,
seconds, default 3x the update interval = 900 s) a typed failure serves
that cache instead: the panel keeps rendering last-good data, entities
stay available, and the outage is logged as a clear warning.  Past the
threshold — or before the first success — the pass fails as before
(typed error logged and chained into UpdateFailed); a later successful
fetch refreshes the cache and the entities recover with no config-entry
reload.  The threshold rides the options flow (validated as a positive
int at least the update interval, with a dedicated error string) and
the strings files; the clock is read through a _monotonic seam so the
staleness tests drive it deterministically.

custom_components/nestquest/__init__.py

@@ -18,7 +18,9 @@ from homeassistant.helpers.event import async_track_time_change
 
 from .const import (
     CONF_ADMIN_USER_IDS,
+    CONF_SNAPSHOT_STALENESS,
     CONF_UPDATE_INTERVAL,
+    DEFAULT_SNAPSHOT_STALENESS,
     DEFAULT_UPDATE_INTERVAL,
     DOMAIN,
     LOGGER,
@@ -518,6 +520,9 @@ async def async_setup_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
             update_interval_seconds=entry.options.get(
                 CONF_UPDATE_INTERVAL, DEFAULT_UPDATE_INTERVAL
             ),
+            staleness_seconds=entry.options.get(
+                CONF_SNAPSHOT_STALENESS, DEFAULT_SNAPSHOT_STALENESS
+            ),
         )
         # The panel event subscription (the SSE task): a background
         # task consumes the API service's transition-event stream and