Split users/internal application and presentation into per-use-case files

The `users` track carries one `user_service.go` and one `users.go` holding every use case — register, login, confirm email, request and confirm password reset, me — rather than a file per use case. The backend reorg flagged this as a real concern during discussion and deferred it explicitly rather than dismissing it (`docs/superpowers/specs/2026-08-28-backend-reorg-design.md`, *Non-goals*).

It matters beyond the `users` track: whatever shape those files settle into is the shape `gonext generate resource` (#15) will emit, and the one every generated project's own domains will copy.

Came out of reviewing the deferred sections of the existing specs.

Read docs/superpowers/specs/2026-08-28-backend-reorg-design.md, especially the Non-goals section, then inspect the users track's user_service.go and users.go. Define the per-use-case file shape for the listed users operations and ensure it is also the shape gonext generate resource (#15) will emit.

Written by the indexing model from the issue text.