Fix settings changes not taking effect until the poll interval elapsed

config.update_screen_settings() writes to config.json immediately, but
each screen's cached check() result only gets refreshed on its own
poll_interval (30 min for weather by default). So e.g. changing
Weather's forecast_days from 3 to 5 saved correctly but kept showing
the old 3-day forecast for up to 30 minutes, since the cached state
was fetched under the old setting and nothing told the scheduler to
re-check sooner.

Added Scheduler.invalidate_screen(), called from every settings route
right after config.update_screen_settings() succeeds, which resets
that screen's cache timestamp so the very next tick re-checks instead
of reusing stale data. Reproduced the bug and verified the fix with a
direct Scheduler unit test, then confirmed end-to-end through a real
running app (POST forecast_days=5, saw it render 5 days within the
next ~5s tick instead of after 30 minutes).

scheduler.py

@@ -88,6 +88,18 @@ class Scheduler:
         self.config.resume_rotation()
         self._wake.set()
 
+    def invalidate_screen(self, screen_id: str):
+        """Force the next tick to re-check() this screen instead of reusing
+        its cached state, however much of its poll_interval is left.
+        Without this, a settings change (e.g. weather's forecast_days) sits
+        in config.json immediately but the screen keeps rendering whatever
+        it last fetched under the *old* settings until its own poll_interval
+        happens to elapse -- up to 30 minutes for weather's default."""
+        cache = self._caches.get(screen_id)
+        if cache is not None:
+            cache.checked_at = 0.0
+        self._wake.set()
+
     def get_status(self) -> dict:
         with self._status_lock:
             status = dict(self._status)