Add AutoBIDSify upload workflow to NeuroJSON

[elainefan331]

Description:
Implement an end-to-end upload workflow that allows users to submit AutoBIDSify-generated JSON documents to a NeuroJSON CouchDB sandbox database for review.
The NeuroJSON backend will act as a secure proxy between the AutoBIDSify converter and CouchDB, keeping CouchDB credentials server-side. The workflow includes both the backend upload API and integration with the AutoBIDSify converter.

Tasks:

• Add CouchDB sandbox upload support in the NeuroJSON backend.
• Store CouchDB credentials securely in backend environment variables.
• Add an API endpoint for AutoBIDSify JSON submissions.
• Add validation, error handling, authentication, and basic abuse protection.
• Test backend-to-CouchDB uploads.
• Add an Upload to NeuroJSON button to the AutoBIDSify converter.
• Connect the converter to the NeuroJSON upload API.
• Test the complete end-to-end upload workflow.

Acceptance Criteria:

• AutoBIDSify-generated JSON documents can be submitted through the NeuroJSON backend.
• Submitted documents are successfully stored in the CouchDB sandbox database.
• CouchDB credentials are never exposed to the AutoBIDSify client.
• Upload success and failure are clearly reported to the user.
• Uploaded documents remain in the sandbox for review before being promoted to production.