Show edge and site versions with Update buttons; Update now for Hammerhead itself

Edges and sites report their version and whether auto-update is on. Hammerhead can ask a machine that
has auto-update on to install the newest release, and shows the outcome. Requests carry an ID so a
stale result can't answer a new one.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

store.go

@@ -208,8 +208,11 @@ type Server struct {
 	limits  map[string][]time.Time
 	oidc    map[string]*oidcDoc
 
-	alerts  map[string]*alertState // active problems by key (see alerts.go)
-	mtaSent map[string]time.Time   // reminder throttle for repeat notifications
+	alerts   map[string]*alertState // active problems by key (see alerts.go)
+	verMu    sync.Mutex             // guards verCache and verAt, the latest releases seen by /api/versions
+	verCache map[string]string
+	verAt    time.Time
+	mtaSent  map[string]time.Time // reminder throttle for repeat notifications
 
 	logs *logStore
 	db   *DB