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/model/model.go

@@ -66,6 +66,24 @@ type Image struct {
 	CreatedAt  time.Time
 }
 
+type LogLevel string
+
+const (
+	LogInfo  LogLevel = "info"
+	LogWarn  LogLevel = "warn"
+	LogError LogLevel = "error"
+)
+
+type ActivityEntry struct {
+	ID          int64
+	HostID      int64
+	OperationID int64
+	Level       LogLevel
+	Message     string
+	Source      string
+	CreatedAt   time.Time
+}
+
 type ServerType struct {
 	Key            string
 	DisplayName    string   `yaml:"display_name"`