[Research Vault] Issue: [Enhancement] Generate & Expose Shared Link for SHARED_LINK Visibility

[KatalKavya96]

Description

Currently, research papers can be saved using the SHARED_LINK visibility mode, but the shareable link is not being generated or exposed properly to the frontend.

Implement the complete shared-link flow so users can securely share papers via unique access links.

Required Work

Backend

• Ensure shareToken exists in Prisma schema (ResearchPaper)
• Generate secure token when visibility is set to SHARED_LINK
• Store token in database
• Return shareToken in API responses
• Add endpoint:

GET /research/shared/:shareToken