fayerman-source stops projects from reaching into each other's files

In a legal document workspace, each project is supposed to be a sealed room: its folders and files belong to it and nothing else. fayerman-source found that a handful of behind-the-scenes operations were trusting folder identifiers without first checking they actually belonged to the project asking for them. That gap could let one project's actions reach across into another's material.

This change adds an ownership check before each risky operation runs. Moving a folder, filing a document into a folder, and deleting a folder are now all confirmed as belonging to the current project first, and clean-up when something is deleted stays confined to that project rather than sweeping across everything. The fix is deliberately narrow and defensive, aimed squarely at keeping projects isolated from one another.