fix(web): SSE 连接添加指数退避重连策略

useAlertSSE hook 和 message store 的 connectSSE 均改为手动重连：
1s→2s→4s→8s→16s→30s(cap)，最大重试 10 次，随机 jitter 0.5-1.0x。
替代浏览器原生 EventSource 固定 ~3s 重连，避免服务端压力。

apps/web/src/stores/message.ts

@@ -1,6 +1,10 @@
 import { create } from 'zustand';
 import { getUnreadCount, listMessages, markRead, type MessageInfo } from '../api/messages';
 
+const INITIAL_DELAY_MS = 1000;
+const MAX_DELAY_MS = 30_000;
+const MAX_RETRIES = 10;
+
 interface MessageState {
   unreadCount: number;
   recentMessages: MessageInfo[];
@@ -71,32 +75,81 @@ export const useMessageStore = create<MessageState>((set, get) => ({
   },
 
   connectSSE: () => {
-    const baseUrl = import.meta.env.VITE_API_BASE_URL || '/api/v1';
-    const token = localStorage.getItem('access_token');
-    if (!token) return () => {};
+    let es: EventSource | null = null;
+    let retryCount = 0;
+    let reconnectTimer: ReturnType<typeof setTimeout> | null = null;
+    let disposed = false;
 
-    const url = `${baseUrl}/messages/stream?token=${encodeURIComponent(token)}`;
-    const es = new EventSource(url);
-
-    es.addEventListener('message', () => {
-      get().fetchUnreadCount();
-      get().fetchRecentMessages();
-    });
-
-    es.addEventListener('alert', () => {
-      get().fetchUnreadCount();
-    });
-
-    es.addEventListener('vital_update', () => {
-      // 体征数据更新事件 — 预留：未来可触发趋势图刷新
-    });
-
-    es.onerror = () => {
-      // SSE 连接断开时 EventSource 会自动重连
+    const clearReconnectTimer = () => {
+      if (reconnectTimer) {
+        clearTimeout(reconnectTimer);
+        reconnectTimer = null;
+      }
     };
 
+    const connect = () => {
+      if (es) {
+        es.close();
+        es = null;
+      }
+      clearReconnectTimer();
+
+      const token = localStorage.getItem('access_token');
+      if (!token || disposed) return;
+
+      const baseUrl = import.meta.env.VITE_API_BASE_URL || '/api/v1';
+      const url = `${baseUrl}/messages/stream?token=${encodeURIComponent(token)}`;
+      es = new EventSource(url);
+
+      es.onopen = () => {
+        retryCount = 0;
+      };
+
+      es.addEventListener('message', () => {
+        get().fetchUnreadCount();
+        get().fetchRecentMessages();
+      });
+
+      es.addEventListener('alert', () => {
+        get().fetchUnreadCount();
+      });
+
+      es.addEventListener('vital_update', () => {
+        // 体征数据更新事件 — 预留：未来可触发趋势图刷新
+      });
+
+      es.onerror = () => {
+        es?.close();
+        es = null;
+
+        retryCount += 1;
+        if (retryCount > MAX_RETRIES || disposed) return;
+
+        const delay = Math.min(
+          INITIAL_DELAY_MS * Math.pow(2, retryCount - 1),
+          MAX_DELAY_MS,
+        );
+        const jitter = delay * (0.5 + Math.random() * 0.5);
+
+        reconnectTimer = setTimeout(() => {
+          reconnectTimer = null;
+          if (disposed) return;
+          const tokenNow = localStorage.getItem('access_token');
+          if (!tokenNow) return;
+          connect();
+        }, jitter);
+      };
+    };
+
+    connect();
+
     return () => {
-      es.close();
+      disposed = true;
+      if (es) {
+        es.close();
+        es = null;
+      }
+      clearReconnectTimer();
     };
   },
 }));