fix(service): Alarm creation without duration - aligns with ai_edge changes

Changes:
1. Modified create_from_mqtt to parse first_frame_time from MQTT data
2. Removed duration_minutes processing logic
3. Set duration_ms=None and last_frame_time=None on alarm creation
4. Updated _determine_alarm_level to handle duration_ms=None (returns level 2 for leave_post)

This ensures alarms are created with status=NEW and no duration/end time,
which will be populated later when the alarm is resolved.

Test: test_alarm_create_no_duration.py validates the new behavior.

Related: Task 2 of alarm status management fix

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

app/services/alarm_event_service.py

@@ -28,9 +28,14 @@ def _determine_alarm_level(alarm_type: str, confidence: float, duration_ms: Opti
     if alarm_type == "intrusion":
         return 3  # 严重
     elif alarm_type == "leave_post":
-        if duration_ms and duration_ms > 30 * 60 * 1000:
+        # 告警触发时 duration_ms 为 None，设置为一般级别
+        if duration_ms is None:
+            return 2  # 一般级别（刚触发，持续时长未知）
+
+        # 根据持续时长判断级别
+        if duration_ms > 30 * 60 * 1000:
             return 3  # 严重
-        elif duration_ms and duration_ms > 10 * 60 * 1000:
+        elif duration_ms > 10 * 60 * 1000:
             return 2  # 一般
         return 1  # 提醒
     elif confidence and confidence > 0.9:
@@ -63,6 +68,15 @@ class AlarmEventService:
             else:
                 event_time = datetime.now(timezone.utc)
 
+            # 解析 first_frame_time（告警首次触发时间）
+            first_frame_str = mqtt_data.get("first_frame_time")
+            first_frame_time = None
+            if first_frame_str:
+                try:
+                    first_frame_time = datetime.fromisoformat(first_frame_str.replace("Z", "+00:00"))
+                except ValueError:
+                    pass
+
             # 置信度保持 float 0-1
             confidence = mqtt_data.get("confidence")
             if confidence is not None:
@@ -71,14 +85,9 @@ class AlarmEventService:
                 if confidence > 1:
                     confidence = confidence / 100.0
 
-            # duration_minutes → duration_ms
-            duration_minutes = mqtt_data.get("duration_minutes")
-            duration_ms = None
-            if duration_minutes is not None:
-                duration_ms = int(float(duration_minutes) * 60 * 1000)
-
             alarm_type = mqtt_data.get("alert_type", "unknown")
-            alarm_level = _determine_alarm_level(alarm_type, confidence or 0, duration_ms)
+            # 告警创建时不传递 duration_ms，传递 None
+            alarm_level = _determine_alarm_level(alarm_type, confidence or 0, None)
 
             alarm = AlarmEvent(
                 alarm_id=alarm_id,
@@ -87,7 +96,9 @@ class AlarmEventService:
                 device_id=mqtt_data.get("camera_id", "unknown"),
                 scene_id=mqtt_data.get("roi_id"),
                 event_time=event_time,
-                duration_ms=duration_ms,
+                first_frame_time=first_frame_time,
+                duration_ms=None,
+                last_frame_time=None,
                 alarm_level=alarm_level,
                 confidence_score=confidence,
                 alarm_status="NEW",