refactor(service): 删除MQTT旧代码 + 修复边缘节点重复显示问题

**删除MQTT旧代码：**
- 删除 mqtt_service.py（已废弃的空壳）
- 从 config.py 删除 MQTTConfig 类和相关配置
- 从 schemas.py 删除 mqtt 字段
- 从 alert_service.py 删除 create_alert_from_mqtt 方法
- 告警上报已改为 HTTP + COS 方案，MQTT机制完全废弃

**修复边缘节点重复显示（方案A）：**
- 清理 edge_devices 表历史数据（删除 edge_device_001、edge_inference_device）
- 禁用 DeviceService 的 handle_heartbeat 自动创建设备功能
- 边缘端未实现心跳机制，告警数从 alarm_event 表统计
- 运行时长、处理帧数字段设为 null（无心跳机制，不可用）
- 添加 count_alarms_by_edge_node 方法统计边缘节点告警数

**影响范围：**
- /admin-api/aiot/edge/device/page 接口返回数据调整
- /admin-api/aiot/edge/device/get 接口返回数据调整
- 确保不破坏现有功能（告警上报已改为HTTP）

app/config.py

@@ -43,20 +43,6 @@ class AIModelConfig:
     api_key: str = ""
 
 
-@dataclass
-class MQTTConfig:
-    """MQTT 配置 (已废弃 - 告警上报已改为 HTTP + COS)"""
-    broker_host: str = "localhost"
-    broker_port: int = 1883
-    client_id: str = "alert_platform"
-    username: str = ""
-    password: str = ""
-    alert_topic: str = "edge/alert/#"
-    heartbeat_topic: str = "edge/alert/heartbeat/#"
-    qos: int = 1
-    enabled: bool = False  # 默认禁用
-
-
 @dataclass
 class RedisConfig:
     """Redis 配置"""
@@ -103,7 +89,6 @@ class Settings(BaseModel):
     cos: COSConfig = COSConfig()
     app: AppConfig = AppConfig()
     ai_model: AIModelConfig = AIModelConfig()
-    mqtt: MQTTConfig = MQTTConfig()
     redis: RedisConfig = RedisConfig()
     camera_name: CameraNameConfig = CameraNameConfig()
 
@@ -137,17 +122,6 @@ def load_settings() -> Settings:
             endpoint=os.getenv("AI_MODEL_ENDPOINT", ""),
             api_key=os.getenv("AI_MODEL_API_KEY", ""),
         ),
-        mqtt=MQTTConfig(
-            broker_host=os.getenv("MQTT_BROKER_HOST", "localhost"),
-            broker_port=int(os.getenv("MQTT_BROKER_PORT", "1883")),
-            client_id=os.getenv("MQTT_CLIENT_ID", "alert_platform"),
-            username=os.getenv("MQTT_USERNAME", ""),
-            password=os.getenv("MQTT_PASSWORD", ""),
-            alert_topic=os.getenv("MQTT_ALERT_TOPIC", "edge/alert/#"),
-            heartbeat_topic=os.getenv("MQTT_HEARTBEAT_TOPIC", "edge/alert/heartbeat/#"),
-            qos=int(os.getenv("MQTT_QOS", "1")),
-            enabled=os.getenv("MQTT_ENABLED", "false").lower() == "true",
-        ),
         redis=RedisConfig(
             host=os.getenv("REDIS_HOST", "localhost"),
             port=int(os.getenv("REDIS_PORT", "6379")),