[Outline] Allow restricting permissions on sub-documents below parent permissions

Document permissions in Outline currently cascade downward: collection-level permissions grant access to all documents, and document-level permissions are copied to all children via sourceId tracking. Users can remove inherited permissions from child documents, but they reappear whenever the parent permissions change (because recreateSourcedMemberships() rebuilds them). There is no way to give a sub-document fewer permissions than its parent. Users have requested the ability to lock down sub-documents so only explicitly granted members can access them, independent of collection or parent document permissions. ### Design Decisions - Framing: Not "restricted" but rather whether a document inherits permissions from its parent/collection. Users toggle inheritance on/off. - Child behavior: Turning off inheritance propagates to all children (bulk write to descendants). This is necessary because hiding a parent from the sidebar would orphan its children. - Sidebar visibility: Document