test(content-node): compose-based integration test for the Kubo client #68

Summary

A CI job that proves the client and node work together, so version bumps are caught.

Estimated size: S

Proposal

• Start the compose kubo service in CI, add content through the Python client, read it back through the gateway, and compare CIDs.

Acceptance criteria

• The job runs on PRs that touch the content node or client and fails on mismatch.

Dependencies

• feat(backend): async Python client for the Kubo RPC API #61

Part of #43. Ground rules for the content layer: no secrets in the repo, content-layer failures must never block or discard quantum execution, and tests and docs ship with every PR.