Cross-tenant isolation audit found:
- retry-psa-push had NO ownership check (CRITICAL) — any user could retry any session's PSA push
- save_task_lane used db.get() without ownership filter, returned 403 revealing existence
- get_session returned 403 instead of 404 for unauthorized access
- stream_documentation returned 403 instead of 404
All now use query-level user_id filtering and return 404 to avoid revealing existence.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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