fix(time-tracking): recalculate duration when start/end time is edited

- TimeTrackingService.update_entry() now calls calculate_duration() when
  start_time or end_time is changed so duration_seconds stays correct.
- Add test_update_entry_recalculates_duration_when_start_end_edited (fixes #451).

app/services/time_tracking_service.py

@@ -383,6 +383,9 @@ class TimeTrackingService:
             entry.start_time = start_time
         if end_time is not None:
             entry.end_time = end_time
+        # Recompute stored duration when start or end time changed
+        if entry.end_time and (start_time is not None or end_time is not None):
+            entry.calculate_duration()
         if notes is not None:
             entry.notes = notes
         if tags is not None:

tests/test_services/test_time_tracking_service_complete.py

@@ -23,6 +23,31 @@ class TestTimeTrackingServiceComplete:
         assert result["entry"].notes == "Updated notes"
         assert result["entry"].billable is False
 
+    def test_update_entry_recalculates_duration_when_start_end_edited(self, app, user, project, time_entry):
+        """Duration should be recomputed when start_time or end_time is updated (issue #451)."""
+        service = TimeTrackingService()
+        old_duration = time_entry.duration_seconds
+
+        # Change start/end so the span is exactly 15 minutes
+        new_start = time_entry.start_time.replace(minute=0, second=0, microsecond=0)
+        new_end = new_start + timedelta(minutes=15)
+        expected_seconds = 15 * 60  # 900
+
+        result = service.update_entry(
+            entry_id=time_entry.id,
+            user_id=user.id,
+            is_admin=False,
+            start_time=new_start,
+            end_time=new_end,
+        )
+
+        assert result["success"] is True
+        assert result["entry"].start_time == new_start
+        assert result["entry"].end_time == new_end
+        assert result["entry"].duration_seconds == expected_seconds, (
+            f"duration_seconds should be {expected_seconds} after editing start/end, got {result['entry'].duration_seconds} (old was {old_duration})"
+        )
+
     def test_update_entry_not_found(self, app, user):
         """Test update with non-existent entry"""
         service = TimeTrackingService()