Make weather update interval configurable, was hardcoded to 30 min

Screen.poll_interval was a fixed class attribute the scheduler read
directly -- added Screen.get_poll_interval(settings) so a screen can make
it settings-driven instead. WeatherScreen overrides it to read a new
"update_minutes" field (default 30, matching the old hardcoded value).
New number input on the Weather settings page.

Caught a real edge case while testing: `raw or 30` treats an explicit 0
as falsy and silently replaces it with 30 instead of clamping to the
1-minute floor. Not reachable via the UI (min="1" on the input) but the
method should still be correct standalone -- fixed with an explicit
None-check instead.

scheduler.py

@@ -180,8 +180,9 @@ class Scheduler:
     def _poll_screen(self, screen_id: str) -> tuple[str, object, bool]:
         cache = self._caches[screen_id]
         screen = self.screens[screen_id]
+        settings = self.config.get_screen_settings(screen_id)
         checked = False
-        if cache.is_stale(screen.poll_interval):
+        if cache.is_stale(screen.get_poll_interval(settings)):
             cache.state = self._check(screen_id)
             cache.checked_at = time.time()
             checked = True