Review use of Enum for job status

[tcompa]

Ref fractal-analytics-platform/fractal-web#318 (comment)

[tcompa]

This was an issue for fractal-server 1.3.14 only, already fixed as part of the 1.4.0 prereleases (e.g. 1.4.0a2).

Refs:

• Review use of Enums #974
• Update CHANGELOG for 1.3.14 #987