feat(backend): add Swagger generation step to backend CI

[GRACENOBLE]

Problem

Swagger docs are generated manually via make swagger and committed. If a developer changes handler annotations but forgets to regenerate, CI passes but the committed docs are stale.

Acceptance criteria

• Add a CI step in backend-ci.yml that runs make swagger and then checks for a clean git diff
• If the diff is non-empty, fail the job with a message directing the developer to run make swagger locally
• Document this requirement in backend/docs/swagger.md

[GRACENOBLE]

Implemented and merged via PR #66.