Merge pull request #71 from theill/api-v1-documents

Add JSON upload API for native clients :sparkles: Add JSON upload API for native clients POST /api/v1/documents creates a document from a multipart file (or files[]) and returns its view_url, edit_url and expires_at. Files over 100 MB go through POST /api/v1/direct_uploads, a CSRF-free mirror of Active Storage's direct uploads, and are attached by signed_ids[]. The controllers inherit from ActionController::API, answer errors as JSON and are rate limited per IP. Documented in README and llms.txt.