[Backend] Create certificate social share links API

[SYLVIANNORUKA]

Description: Implement API to generate shareable links and LinkedIn-ready metadata for earned certificates.

Requirements:

• Handle share link generation requests
• Track share link view counts
• Validate certificate is not revoked before sharing
• Manage Open Graph metadata for link previews
• Provide LinkedIn add-to-profile URL

Acceptance Criteria:

• Generation requests are handled
• View counts are tracked
• Revocation status is validated before sharing
• Open Graph metadata works
• LinkedIn URL is provided

Technical Details:

Files: backend/src/modules/certificates/certificate-share.controller.ts, backend/src/modules/certificates/certificate-share.service.ts
Libraries: @nestjs/common, prisma