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>

manifests/hub.yaml

@@ -117,7 +117,7 @@ spec:
     spec:
       containers:
         - name: hub
-          image: gitea.dooplex.hu/admin/felhom-hub:v0.3.6
+          image: gitea.dooplex.hu/admin/felhom-hub:v0.3.7
           ports:
             - containerPort: 8080
               name: http