feat: add private project access

Co-Authored-By: Paperclip <noreply@paperclip.ing>

doc/DATABASE.md

@@ -167,6 +167,8 @@ These rows are company-scoped and user-scoped. A missing row means the user is j
 
 Both tables use a unique key on `(company_id, user_id, resource_id)` and keep `state` as `joined` or `left`. Join/leave mutations are idempotent board-user `/me` operations and write activity entries when the effective state changes.
 
+Private-project authorization uses the separate `project_access_members` table. Its user/agent rows are security grants, not sidebar preferences, and are evaluated by the same issue-read predicate as issue-level grants. Do not merge or overload these two concepts.
+
 ## Decision training snapshot retention
 
 `decision_training_examples` stores a point-in-time copy of an issue, its comments, relevant runs, and the selected decision. Each row carries the `scrub_deleted_comments_v1` retention policy marker, and JSONL exports include that marker alongside the snapshot.