#357 feat: POST /api/sandboxes/files - file upload endpoint

Summary

• Implements POST /api/sandboxes/files endpoint for uploading files to GitHub org submodules
• Accepts multipart form data with target path and files
• Auth via validateAuthContext, resolves sandbox repo from account snapshot
• Commits files via GitHub Contents API with submodule path resolution
• Zod validation for request body

Related

• Paperclip: REC-31 (API sub-issue of REC-29)

Test plan

• Upload single file to sandbox repo
• Upload multiple files
• Verify submodule path resolution
• Auth validation (unauthorized requests rejected)
• Invalid path handling

Summary by CodeRabbit

• New Features
  • Added authenticated API endpoint for uploading files to sandboxes, supporting multipart form submissions with optional custom paths and commit messages
  • Implemented CORS preflight request handling for browser compatibility
  • Integrated GitHub repository file creation and update operations
  • Included comprehensive request validation and error handling with partial upload support