Per-user token isolation for result access control

[Par-t]

Problem

All users in an instance share a single user_token. Any user who knows another user's submission_id can read their result via GET /results/{id}.

Solution

1. External auth layer issues a unique token per user instead of one shared token per instance
2. TEE service tracks ownership: token → set(submission_ids) per instance
3. GET /results/{id} enforces: token must own the submission_id, otherwise returns 403
4. Users can hold memberships across multiple instances

Open design questions

• What should admin see? Current architecture gives admin full result access, but the TEE trust model may require restricting this (e.g., only aggregated stats, not per-submission scores)
• Should admin access be configurable per skill via the SkillCard?

Relevant code

• api/routes.py — _tokens dict, GET /results/{id}
• tests/test_e2e.py — documented limitation comment