Missing problem constructors for common statuses (Unauthorized, Forbidden, Conflict, Unprocessable, Internal) #16

problem_helpers.go ships ProblemBadRequest/NewBadRequestProblem and ProblemNotFound/NewNotFoundProblem, but nothing for the other statuses that come up just as often in a typical API:

• 401 Unauthorized
• 403 Forbidden
• 409 Conflict
• 422 Unprocessable Entity
• 500 Internal Server Error

Without these, every call site falls back to the fully manual httpsuite.NewProblemDetails(status, type, title, detail) — which is exactly the 4-argument ceremony the NewBadRequestProblem/NewNotFoundProblem helpers exist to avoid for those two statuses.

Measured impact in one consumer (GolgiMed, ~150k LOC): 845 manual NewProblemDetails(...) calls, of which ~730 are exactly the canonical (status, "unauthorized", "Unauthorized", detail) / (status, "forbidden", "Forbidden", detail) / etc. shape that a NewUnauthorizedProblem(detail) / NewForbiddenProblem(detail) would collapse to one line.

Requested: ProblemUnauthorized/NewUnauthorizedProblem, ProblemForbidden/NewForbiddenProblem, ProblemConflict/NewConflictProblem, ProblemUnprocessable/NewUnprocessableProblem, ProblemInternal/NewInternalProblem, matching the existing ProblemBadRequest/ProblemNotFound pattern.