test(backend): WebMvcTests for six controllers

[CMaintz]

59 tests for the Job, Company, User, Skill, ProfileSection and InterviewPrep controllers, same setup as ApplicationControllerTest. 595 tests total, all passing locally. Stacked on #157.

Two things I noticed but didn't fix here:

• Outreach "not found" returns 400, while the OpenAPI docs say 404.
• Framework errors (missing param, bad enum, malformed JSON) hit the catch-all handler and return 500. The tests don't lock that in, so fixing it later won't break anything.