task 4: DRY permission checking by extracting checkSetPermission helper

Consolidates the duplicated user-admin, explicit permission, and
set.Permissions loop logic from verifyAccess, verifyModifyAccess,
and verifySetModifyAccess into a single checkSetPermission helper.

- checkSetPermission accepts a requiredRole; empty string means any role.
- verifyAccess delegates set-level permission check to helper.
- verifyModifyAccess delegates owner-only check to helper.
- verifySetModifyAccess delegates directly to helper.

All tests pass with -race -cover.

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