fix: Sentinel SSE 每次轮询都推送 (实时性) + isolation 默认 false + 文档

Sentinel:
- poll_interval 3s→2s, 每次轮询都推 SSE (不再等状态变化)
- 解决稳定状态下数据更新不实时问题

插件系统:
- 默认 isolation: false (避免插件 WebUI 不显示)
- example_plugin 显式设置 isolation: false
- 插件开发指南: settings 示例增加 isolation 说明

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

plugins/sentinel/__init__.py

@@ -175,23 +175,18 @@ class Plugin(PluginWebMixin):
     async def _poll_loop(self, interval: int):
         while self._running:
             nodes = self._get_nodes()
-            changed = False
             for node in nodes:
                 if not node.get("enabled", True):
                     continue
-                prev = _NODE_CACHE.get(node.get("id", ""), {})
                 online, sys_info, err = await self._fetch_system(node)
-                new_status = {
+                _NODE_CACHE[node.get("id", "")] = {
                     "online": online,
                     "system": sys_info or {},
                     "last_seen": time.time(),
                     "error": err if not online else "",
                 }
-                if (prev.get("online") != online
-                        or abs(prev.get("last_seen", 0) - new_status["last_seen"]) > 5):
-                    changed = True
-                _NODE_CACHE[node.get("id", "")] = new_status
-            if changed:
+            # 每次轮询都推送 SSE (确保实时性)
+            if nodes:
                 await self.push_sse_event("nodes_update", {"nodes": self._build_cache_list()})
             await asyncio.sleep(interval)