feat(go-api): implement tenant member management (issue #15294) (#15295)

## Summary

Ports the Python `tenant_api` team/member management endpoints to Go,
adding 4 endpoints under `/api/v1/tenants/:tenant_id/`:

- `GET /tenants/:tenant_id/users` — list non-owner members with user
details (owner only)
- `POST /tenants/:tenant_id/users` — invite a user by email; creates
invite-role join record (owner only)
- `DELETE /tenants/:tenant_id/users` — remove a member by `user_id`;
owner can remove anyone, members can remove themselves
- `PATCH /tenants/:tenant_id` — accept a pending invitation,
transitioning role `invite → normal`

Closes #15294