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/Dockerfile

@@ -23,6 +23,9 @@ COPY --from=builder /felhom-hub /usr/local/bin/felhom-hub
 
 RUN mkdir -p /data /etc/felhom-hub
 
+# Seed assets: baked into image, copied to PVC on first run
+COPY assets/ /usr/share/felhom/assets-seed/
+
 ENV TZ=Europe/Budapest
 
 EXPOSE 8080