Add upload workflow to NeuroJSON

[elainefan331]

Description:
Add an authenticated upload workflow for submitting JSON datasets to the NeuroJSON sandbox for review.

Users will upload through a login-protected /upload page on NeuroJSON.io. AutoBIDSify will provide a button that opens this page in the browser.

Tasks:

• Add CouchDB sandbox upload support in the NeuroJSON backend.
• Require NeuroJSON login for uploads.
• Add uploader metadata, validation, and rate limiting.
• Test backend-to-CouchDB uploads.
• Add a /upload page with a .json file picker.
• Add an Upload to NeuroJSON.io button in AutoBIDSify.
• Test the complete end-to-end upload workflow.

Acceptance Criteria:

• Only logged-in users can upload.
• Uploaded JSON documents are stored in the sandbox database.
• Uploader information is recorded.
• Upload success/failure is shown clearly.
• AutoBIDSify opens the NeuroJSON upload page.