[P1] Add integration tests for every `client/app/api/subscriptions/*` route

[Calebux]

Summary

Subscription CRUD and pause/resume/import endpoints are critical and should have route-level coverage.

Scope

• Area: client-api
• Priority: P1
• Backlog ID: #36

Acceptance Criteria

• Tests cover list, create, update, delete, pause, resume, and import flows.
• Authz failure paths are included.
• Contract between route and service layer is documented.

Implementation Notes

• Keep the implementation aligned with current architecture decisions and security constraints in this repo.
• Include tests for changed behavior and update docs where relevant.

Definition of Done

• Acceptance criteria met
• Tests added/updated and passing
• Documentation updated
• No security regressions introduced

Links

• Backlog source: docs/repo-issue-backlog-2026-05.md