feat(hub): asset management API with PVC storage and image seed

Add internal/assets package that manages app assets (logos, screenshots)
on Hub PVC with automatic seeding from baked-in image copy on first run.
Two new API endpoints: GET /assets/manifest (JSON with SHA-256 checksums)
and GET /assets/file/{name} for controllers to sync assets.

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

hub/cmd/hub/main.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"gitea.dooplex.hu/admin/felhom-hub/internal/api"
+	"gitea.dooplex.hu/admin/felhom-hub/internal/assets"
 	"gitea.dooplex.hu/admin/felhom-hub/internal/monitor"
 	"gitea.dooplex.hu/admin/felhom-hub/internal/notify"
 	"gitea.dooplex.hu/admin/felhom-hub/internal/store"
@@ -117,12 +118,17 @@ func main() {
 		logger.Printf("[INFO] Template fetcher started (every %s)", cfg.Registry.TemplateInterval)
 	}
 
+	// Initialize asset manager (PVC storage with image seed)
+	assetsDir := filepath.Join(cfg.Server.DataDir, "assets")
+	assetsMgr := assets.New(assetsDir, "/usr/share/felhom/assets-seed", logger)
+
 	// Initialize handlers — pass templateFetcher as interface (nil-safe)
 	var templateProvider api.ConfigTemplateProvider
 	if templateFetcher != nil {
 		templateProvider = templateFetcher
 	}
 	apiHandler := api.New(dataStore, cfg.API.ReportAPIKey, cfg.Notifications.ResendAPIKey, cfg.Notifications.FromEmail, templateProvider, logger)
+	apiHandler.SetAssetManager(assetsMgr)
 
 	// Initialize notification dispatcher
 	dispatcher := notify.NewDispatcher(