Create share.service.ts with token generation (128-bit base64url),
CRUD operations, validation, and visibility transition side effects.
Add share endpoints under /api/setups/:id/shares, shared access at
/api/shared/:token, and /s/:token short URL redirect.
Plan: 32-02 (Setup Sharing System - Share Link Backend)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
