Minor cleanup in update.py

- Move datetime import to module level
- Replace assert on load_cached_launches with or {} fallback so the
  updater doesn't crash when the cache file is absent
- Condense upcoming-trips list comprehension to one line

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

update.py

@@ -29,6 +29,9 @@ from agenda.types import StrDict
 from web_view import app
 
 
+from datetime import date, timedelta
+
+
 async def update_bank_holidays(config: flask.config.Config) -> None:
     """Update cached copy of UK Bank holidays."""
     t0 = time()
@@ -318,8 +321,7 @@ def update_thespacedevs(config: flask.config.Config) -> None:
     """
     rocket_dir = os.path.join(config["DATA_DIR"], "thespacedevs")
 
-    existing_data = agenda.thespacedevs.load_cached_launches(rocket_dir)
-    assert existing_data
+    existing_data = agenda.thespacedevs.load_cached_launches(rocket_dir) or {}
 
     if agenda.thespacedevs.is_launches_cache_fresh(rocket_dir):
         return
@@ -411,16 +413,13 @@ def update_gandi(config: flask.config.Config) -> None:
 
 def update_weather(config: flask.config.Config) -> None:
     """Refresh weather cache for home and all upcoming trips."""
-    from datetime import date, timedelta
 
     today = date.today()
     forecast_window = today + timedelta(days=8)
 
     trips = agenda.trip.build_trip_list()
     upcoming = [
-        t
-        for t in trips
-        if (t.end or t.start) >= today and t.start <= forecast_window
+        t for t in trips if (t.end or t.start) >= today and t.start <= forecast_window
     ]
 
     seen: set[tuple[float, float]] = set()