fix: 管理后台连接状态 + 修复按钮无响应

连接状态（正式）：
- /api/display/state 扩展：screens_online（大屏心跳统计，opc/display/heartbeat）+ last_command（最近指令与发布结果）
- 前端大屏每 10s 上报心跳（稳定 client_id），后端 30s TTL 统计在线大屏数
- 管理台新增「连接状态」卡片：后端/MQTT/在线大屏/最近指令，3s 轮询刷新

按钮无响应根治：
- admin.js 改为 document 事件委托（data-nav/data-control/data-action/复制/预览），不再依赖逐个绑定
- /static 与 /admin 响应加 no-cache 头，杜绝旧 JS/CSS 缓存导致的按钮失效
- SSE 不再自动重载页面（避免与操作回调双重重载清掉反馈）；立即播放改为 toast 不重载

backend/app/mqtt.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
-"""MQTT 发布中心 —— 后端 → 前端控制通道
-   页面控制 / 媒体控制 / 卡片展示指令统一走 opc/display/command
-   数据快照按 DPM_MQTT_TICK_INTERVAL 推送 opc/dashboard/tick
+"""MQTT 发布/订阅中心 —— 后端 ↔ 前端控制通道
+   发布：opc/display/command（页面/媒体/卡片控制）、opc/dashboard/tick（数据快照）
+   订阅：opc/display/heartbeat（大屏在线心跳，统计在线大屏数）
 """
 
 import json
@@ -27,6 +27,8 @@ class MqttHub:
         self.client = None
         self.connected = False
         self._lock = threading.RLock()
+        self._screens = {}            # client_id -> last_seen_ts
+        self.last_command = None      # {action, params, ts, published}
 
     # ---------- 生命周期 ----------
     def start(self):
@@ -43,6 +45,7 @@ class MqttHub:
                 self.client.username_pw_set(settings.MQTT_USERNAME, settings.MQTT_PASSWORD)
             self.client.on_connect = self._on_connect
             self.client.on_disconnect = self._on_disconnect
+            self.client.on_message = self._on_message
             self.client.connect_async(settings.MQTT_HOST, settings.MQTT_PORT, keepalive=30)
             self.client.loop_start()
             log.info("MQTT 连接中 %s:%s ...", settings.MQTT_HOST, settings.MQTT_PORT)
@@ -62,6 +65,7 @@ class MqttHub:
         if rc == 0:
             self.connected = True
             log.info("MQTT 已连接 %s:%s", settings.MQTT_HOST, settings.MQTT_PORT)
+            client.subscribe(settings.TOPIC_HEARTBEAT, qos=0)
         else:
             log.warning("MQTT 连接失败 rc=%s", rc)
 
@@ -70,6 +74,34 @@ class MqttHub:
         if rc != 0:
             log.warning("MQTT 断开（rc=%s），自动重连中...", rc)
 
+    def _on_message(self, client, userdata, msg):
+        """接收大屏心跳：记录 client_id 与时间"""
+        if msg.topic == settings.TOPIC_HEARTBEAT:
+            try:
+                payload = json.loads(msg.payload.decode("utf-8"))
+                cid = payload.get("client_id") or msg.topic
+                with self._lock:
+                    self._screens[cid] = time.time()
+                    # 清理过期
+                    cutoff = time.time() - settings.SCREEN_TTL
+                    self._screens = {k: v for k, v in self._screens.items() if v > cutoff}
+            except Exception:  # noqa: BLE001
+                pass
+
+    # ---------- 状态查询 ----------
+    def screens_online(self):
+        with self._lock:
+            cutoff = time.time() - settings.SCREEN_TTL
+            return sum(1 for v in self._screens.values() if v > cutoff)
+
+    def status(self):
+        return {
+            "mqtt_connected": self.connected,
+            "mqtt_host": f"{settings.MQTT_HOST}:{settings.MQTT_PORT}",
+            "screens_online": self.screens_online(),
+            "last_command": self.last_command,
+        }
+
     # ---------- 发布 ----------
     def publish(self, topic, payload, qos=1, retain=False):
         if not self.client or not self.connected:
@@ -93,6 +125,13 @@ class MqttHub:
         }
         ok = self.publish(settings.TOPIC_COMMAND, payload)
         payload["published"] = ok
+        with self._lock:
+            self.last_command = {
+                "action": action,
+                "params": params or {},
+                "ts": payload["ts"],
+                "published": ok,
+            }
         if ok:
             bus.emit(payload)  # SSE 兼容通道（仅发布成功时推送）
         return payload