feat(controller): Hub asset syncer for logos and screenshots

Add internal/assets package that downloads and caches app assets from
Hub API with SHA-256 change detection. Assets resolve from synced cache
first, falling back to baked-in directory. Daily sync schedule +
on-demand POST /api/assets/sync endpoint.

Config: assets.sync_enabled + assets.sync_schedule (default 05:00)

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

controller/internal/web/server.go

@@ -11,6 +11,7 @@ import (
 	"sync"
 	"time"
 
+	"gitea.dooplex.hu/admin/felhom-controller/internal/assets"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/backup"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/config"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/monitor"
@@ -61,6 +62,9 @@ type Server struct {
 
 	// Hub push status callback — set via SetHubPushStatus for monitoring page
 	hubPushStatusFn func() HubPushStatusData
+
+	// Asset syncer for Hub-managed assets (optional)
+	assetsSyncer *assets.Syncer
 }
 
 func NewServer(cfg *config.Config, stackMgr *stacks.Manager, cpuCollector *system.CPUCollector, backupMgr *backup.Manager, crossDrive *backup.CrossDriveRunner, sched *scheduler.Scheduler, sett *settings.Settings, alertMgr *AlertManager, notif *notify.Notifier, updater *selfupdate.Updater, logger *log.Logger, version string) *Server {
@@ -134,6 +138,11 @@ func (s *Server) SetHubPushStatus(fn func() HubPushStatusData) {
 	s.hubPushStatusFn = fn
 }
 
+// SetAssetsSyncer sets the Hub asset syncer for resolving app assets.
+func (s *Server) SetAssetsSyncer(as *assets.Syncer) {
+	s.assetsSyncer = as
+}
+
 // InRestoreMode returns true if the server is in DR restore mode.
 func (s *Server) InRestoreMode() bool {
 	s.restoreMu.RLock()
@@ -286,7 +295,13 @@ const assetsDir = "/usr/share/felhom/assets"
 
 func (s *Server) serveAsset(w http.ResponseWriter, r *http.Request, filename string) {
 	filename = filepath.Base(filename)
-	path := filepath.Join(assetsDir, filename)
+
+	var path string
+	if s.assetsSyncer != nil {
+		path = s.assetsSyncer.Resolve(filename)
+	} else {
+		path = filepath.Join(assetsDir, filename)
+	}
 
 	if _, err := os.Stat(path); os.IsNotExist(err) {
 		http.NotFound(w, r)