fix age to start at 0 on start date of cycle instead of 1

backend/apps/farms/models.py

@@ -116,13 +116,15 @@ class Cycle(models.Model):
 
     @property
     def current_day(self) -> int:
+        """0-based cycle age: start_date is day 0."""
         from apps.operations.services.visibility import visible_through_date
 
         effective = visible_through_date()
         if effective < self.start_date:
             return 0
-        elapsed = (effective - self.start_date).days + 1
-        return min(max(elapsed, 1), self.total_days)
+        elapsed = (effective - self.start_date).days
+        max_day = max(self.total_days - 1, 0)
+        return min(max(elapsed, 0), max_day)
 
     @property
     def visible_through_date(self) -> date: