Extract the duplicated user-search business logic into two helpers both API
versions call, and refactor v1's handlers onto them:
- user.SearchUsers wraps ListUsers + email obfuscation (global search)
- models.SearchUsersForProject wraps the project read check + ListUsersFromProject
Each handler keeps its own forbidden mapping (v1 echo.ErrForbidden vs v2
huma) so v1 stays byte-identical on the wire.