feat: 多主机纳管、用户认证、noVNC控制台、深色主题

主要功能:
- 多主机管理: 支持TCP/SSH方式纳管远程KVM主机
- 用户认证: JWT token认证, 默认admin/admin123
- noVNC控制台: 前端集成noVNC, WebSocket代理VNC连接
- 深色主题: 全局Element Plus深色主题覆盖
- 虚拟机操作: 克隆、迁移、XML编辑、快照管理
- 资源监控: CPU/内存/磁盘IO/网络流量实时监控

Bug修复:
- libvirt getInfo()内存单位修正(MiB非KiB)
- 远程主机VNC 0.0.0.0监听地址连接策略修复
- Dashboard定时器内存泄漏修复
- bcrypt版本兼容性修复

backend/app/libvirt_conn.py

@@ -1,38 +1,40 @@
-"""libvirt 连接管理 - 使用连接池模式"""
+"""libvirt 多主机连接管理器"""
 import libvirt
 from contextlib import contextmanager
-from app.config import settings
 import logging
+from app.hosts import get_host, update_host_status, list_hosts
 
 logger = logging.getLogger(__name__)
 
 
 class LibvirtConnection:
-    """libvirt 连接管理器（只读连接保持，写操作用短连接）"""
+    """单个 libvirt 连接管理器（只读保持，读写短连接）"""
 
-    def __init__(self):
+    def __init__(self, uri: str, host_id: str = "local"):
+        self._uri = uri
+        self._host_id = host_id
         self._conn = None
 
     def _connect(self):
-        """建立新的 libvirt 连接"""
+        """建立只读连接"""
         try:
-            conn = libvirt.openReadOnly(settings.LIBVIRT_URI)
+            conn = libvirt.openReadOnly(self._uri)
             if conn is None:
-                raise ConnectionError(f"无法连接到 libvirt: {settings.LIBVIRT_URI}")
+                raise ConnectionError(f"无法连接到 libvirt: {self._uri}")
             return conn
         except libvirt.libvirtError as e:
-            logger.error(f"libvirt 连接错误: {e}")
+            logger.error(f"libvirt 连接错误 ({self._host_id}): {e}")
             raise
 
     def _connect_rw(self):
         """建立可读写连接"""
         try:
-            conn = libvirt.open(settings.LIBVIRT_URI)
+            conn = libvirt.open(self._uri)
             if conn is None:
-                raise ConnectionError(f"无法连接到 libvirt (RW): {settings.LIBVIRT_URI}")
+                raise ConnectionError(f"无法连接到 libvirt (RW): {self._uri}")
             return conn
         except libvirt.libvirtError as e:
-            logger.error(f"libvirt RW 连接错误: {e}")
+            logger.error(f"libvirt RW 连接错误 ({self._host_id}): {e}")
             raise
 
     @property
@@ -73,6 +75,79 @@ class LibvirtConnection:
             "cpu_speed": c.getInfo()[3],  # MHz
         }
 
+    def close(self):
+        """关闭连接"""
+        if self._conn is not None:
+            try:
+                self._conn.close()
+            except Exception:
+                pass
+            self._conn = None
 
-# 全局单例
-libvirt_conn = LibvirtConnection()
+
+class ConnectionPool:
+    """多主机连接池"""
+
+    def __init__(self):
+        self._pool: dict[str, LibvirtConnection] = {}
+
+    def get(self, host_id: str = "local") -> LibvirtConnection:
+        """获取指定主机的连接管理器"""
+        if host_id not in self._pool:
+            host = get_host(host_id)
+            if not host:
+                raise ValueError(f"主机 '{host_id}' 不存在")
+            self._pool[host_id] = LibvirtConnection(host.uri, host_id)
+        return self._pool[host_id]
+
+    def remove(self, host_id: str):
+        """移除并关闭指定主机的连接"""
+        if host_id in self._pool:
+            self._pool[host_id].close()
+            del self._pool[host_id]
+
+    def get_conn(self, host_id: str = "local"):
+        """快捷获取 libvirt 只读连接"""
+        return self.get(host_id).conn
+
+    @contextmanager
+    def get_rw(self, host_id: str = "local"):
+        """快捷获取 libvirt 读写连接"""
+        mgr = self.get(host_id)
+        with mgr.get_rw() as conn:
+            yield conn
+
+    def refresh_all(self):
+        """刷新所有连接状态"""
+        for host_id, mgr in list(self._pool.items()):
+            try:
+                alive = mgr.conn.isAlive()
+                update_host_status(host_id, "online" if alive else "offline")
+            except Exception:
+                update_host_status(host_id, "offline")
+                mgr.close()
+
+
+# 全局连接池单例
+conn_pool = ConnectionPool()
+
+# 保持向后兼容：本机连接的直接引用
+libvirt_conn = property(lambda self: conn_pool.get("local"))
+
+
+class _CompatConn:
+    """向后兼容包装，让现有 `libvirt_conn.conn` 等调用继续工作"""
+
+    @property
+    def conn(self):
+        return conn_pool.get_conn("local")
+
+    def get_rw(self):
+        return conn_pool.get_rw("local")
+
+    def get_host_info(self):
+        return conn_pool.get("local").get_host_info()
+
+
+# 全局兼容单例
+libvirt_conn = _CompatConn()