feat: filespace document backend

[zqxxxxx]

Summary

Track the Filespace document backend work for octo-server.

Scope

• Add document backend module, migrations, service/API layer, and module registration.
• Add local signed file storage support for Filespace document upload/download flows.
• Add document results to global search.
• Register localized document/file error codes and keep document routes behind auth, space middleware, and shared UID rate limiting.

Spec

• .octospec/tasks/filespace-documents-backend/brief.md

PR

• feat: add filespace document backend #552