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/config/config.go

@@ -138,7 +138,9 @@ type LoggingConfig struct {
 }
 
 type AssetsConfig struct {
-	SourceURL string `yaml:"source_url"` // Only used during build, not runtime
+	SourceURL    string `yaml:"source_url"`     // Only used during build, not runtime
+	SyncEnabled  bool   `yaml:"sync_enabled"`   // Download assets from Hub API
+	SyncSchedule string `yaml:"sync_schedule"`  // Daily sync time (HH:MM), default "05:00"
 }
 
 type HubConfig struct {
@@ -263,6 +265,7 @@ func applyDefaults(cfg *Config) {
 	di(&cfg.Logging.MaxSizeMB, 10)
 	di(&cfg.Logging.MaxFiles, 3)
 	d(&cfg.Assets.SourceURL, "https://felhom.eu")
+	d(&cfg.Assets.SyncSchedule, "05:00")
 	di(&cfg.System.ReservedMemoryMB, 384)
 }