fix(nodes): stop a restarting panel from reporting itself as down

Adding a node fails right after that node's panel restarts. nodes/add
probes the node's /panel/api/server/status first, and that endpoint
returns whatever the @2s ticker last sampled - nil until the first tick
lands, so the master reads a healthy panel as unreachable and rejects it
with "Add node (remote returned success=false: )", an error whose
message is empty because the node answered success with a null obj.

The window is far wider than one tick: GetStatus resolved the public
IPv4/IPv6 addresses inline and held s.mu across every lookup, so a box
with no IPv6 route spent 3s per service - about 15s of nil status after
each restart, and the same stall on a fresh panel's first sample.

- status now answers from CurrentStatus, which samples on demand when
  the ticker has not run yet instead of returning a null obj
- the public-IP lookups run in the background and outside s.mu, so a
  status sample never waits on them
- probe tells "no status yet" apart from a genuine success=false, so the
  master's error says something when it meets an older node

internal/web/controller/server.go

@@ -106,7 +106,7 @@ func (a *ServerController) startTask() {
 }
 
 // status returns the current server status information.
-func (a *ServerController) status(c *gin.Context) { jsonObj(c, a.serverService.LastStatus(), nil) }
+func (a *ServerController) status(c *gin.Context) { jsonObj(c, a.serverService.CurrentStatus(), nil) }
 
 func (a *ServerController) getFail2banStatus(c *gin.Context) {
 	jsonObj(c, a.serverService.GetFail2banStatus(), nil)