-- Private structures. A structure is public by default (private = 0); when
-- private = 1 only the users listed in structure_members may reach its routes,
-- mount its dbs, or open it in the workshop — a flat set of equals, no creator
-- or owner. Enforcement lives in the platform (index.js / plumbing.js), never in
-- user code — see canAccessStructure in db.js.
ALTER TABLE structures ADD COLUMN private INTEGER NOT NULL DEFAULT 0;

CREATE TABLE structure_members (
    structure_id INTEGER NOT NULL,
    user_id INTEGER NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    PRIMARY KEY(structure_id, user_id),
    FOREIGN KEY(structure_id) REFERENCES structures(id),
    FOREIGN KEY(user_id) REFERENCES users(id)
);