fix(aiot): 告警触发时不发送持续时长，等待回岗后再计算

修改算法层告警触发逻辑：
- 移除 duration_minutes 字段（告警触发时不计算持续时长）
- 新增 first_frame_time 字段（记录离开时间）
- 修改消息为固定文本"人员离岗告警"

持续时长改为在人员回岗时由 alarm_resolve 事件计算

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

algorithms.py

@@ -289,16 +289,14 @@ class LeavePostAlgorithm:
                    (current_time - self.alert_cooldowns[cooldown_key]).total_seconds() > self.cooldown_sec:
 
                     bbox = self._get_latest_bbox(tracks, roi_id)
-                    total_off_duty_sec = (current_time - self._leave_start_time).total_seconds()
-                    elapsed_minutes = int(total_off_duty_sec / 60)
 
                     alerts.append({
                         "track_id": roi_id,
                         "camera_id": camera_id,
                         "bbox": bbox,
-                        "duration_minutes": elapsed_minutes,
                         "alert_type": "leave_post",
-                        "message": f"离岗 {elapsed_minutes} 分钟",
+                        "message": "人员离岗告警",
+                        "first_frame_time": self._leave_start_time.strftime('%Y-%m-%d %H:%M:%S'),
                     })
 
                     self.alert_cooldowns[cooldown_key] = current_time