Add activity log system for provisioning lifecycle visibility

Hosts stuck in states like pxe_ready had zero visibility into why.
This adds a persistent activity log that records every meaningful
step (state transitions, PXE events, cluster join stages, failures)
and surfaces it on the host detail page with live SSE updates.
Includes a stuck-detection warning banner when a host sits in
pxe_ready for >10 minutes with no iPXE request.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/hosts.go

@@ -124,6 +124,7 @@ func (a *HostAPI) Rebuild(w http.ResponseWriter, r *http.Request) {
 		writeJSONErr(w, http.StatusInternalServerError, "failed to acquire lock")
 		return
 	}
+	a.Runner.LogActivity(r.Context(), host.ID, model.LogInfo, "api", "Rebuild triggered via API")
 
 	if err := a.Orchestrator.PrepareRebuild(r.Context(), host.ID); err != nil {
 		_ = a.Locks.Release(r.Context(), host.ID)