Change stale data alert threshold from 4 to 12 hours

- Stale data alerts now only trigger after 12 hours without new data
- Reduces false alerts during expected data gaps

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

src/alerting.py

@@ -282,7 +282,7 @@ class WaterLevelAlertSystem:
 
         return alerts
 
-    def check_data_freshness(self, max_age_hours: int = 4) -> List[WaterAlert]:
+    def check_data_freshness(self, max_age_hours: int = 12) -> List[WaterAlert]:
         """Check if data is fresh enough"""
         alerts = []