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>

cmd/provisioning/main.go

@@ -54,16 +54,18 @@ func main() {
 	ops := &store.Operations{DB: database}
 	locks := &store.Locks{DB: database, TTLMinutes: cfg.Locks.TTLMinutes}
 	images := &store.Images{DB: database}
+	activity := &store.Activity{DB: database}
 
 	imageSvc := &image.Service{Store: images, ImageDir: cfg.Images.Dir}
 
 	hub := events.NewHub()
 
 	runner := &orchestrator.Runner{
-		Hosts: hosts,
-		Ops:   ops,
-		Locks: locks,
-		Hub:   hub,
+		Hosts:    hosts,
+		Ops:      ops,
+		Locks:    locks,
+		Hub:      hub,
+		Activity: activity,
 	}
 
 	pxeSupervisor := pxe.NewSupervisor(pxe.SupervisorConfig{
@@ -126,6 +128,7 @@ func main() {
 		Ops:          ops,
 		Locks:        locks,
 		Images:       images,
+		Activity:     activity,
 		ImageSvc:     imageSvc,
 		Runner:       runner,
 		Orchestrator: hostOrch,