- Backend create/can validation paths emit machine-readable error codes
(slug_taken, slug_invalid, content_empty, content_too_large,
expiry_invalid, rate_limited, ...) alongside the human message
- new.html JS maps codes to plain-language guidance with generic fallback
- Result card colored via --ok/--err/--warn left border (result-ok/err/warn)
- docs/API.md error section documents the code field
- Tests assert the code on every validation path
