feat(proxmox): add tail endpoint and enhance journalctl with multi-service support

Add new `/proxmox/tail` API endpoint for streaming file contents from Proxmox
nodes and LXC containers via WebSocket. Extend journalctl endpoint to support
filtering by multiple services simultaneously.

Changes:
- Add `GET /proxmox/tail` endpoint supporting node-level and LXC container file tailing
- Change `service` parameter from string to array in journalctl endpoints
- Add input validation (`checkValidInput`) to prevent command injection
- Refactor command formatting with proper shell quoting

Security: All command inputs are validated for dangerous characters before

internal/proxmox/node.go

@@ -14,6 +14,7 @@ type NodeConfig struct {
 	VMID     *int     `json:"vmid"` // unset: auto discover; explicit 0: node-level route; >0: lxc/qemu resource route
 	VMName   string   `json:"vmname,omitempty"`
 	Services []string `json:"services,omitempty" aliases:"service"`
+	Files    []string `json:"files,omitempty" aliases:"file"`
 } // @name ProxmoxNodeConfig
 
 type Node struct {