feat: member portal Sprint 2 — digest history page and restore Digest/DigestConfig schema models

- Restore Digest + DigestConfig models to schema.prisma (wiped by prior git reset)
- Add Tenant.digestConfig / Tenant.digests relations
- GET /my/digest API endpoint (last 20 digests, truncated summary)
- BFF route /api/my/digest
- /my/digest page: digest cards with date, message count, summary preview
- Add Digest nav item to MemberShell sidebar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/app/my/layout.tsx

@@ -4,6 +4,7 @@ import { redirect } from 'next/navigation';
 
 const NAV = [
   { href: '/my', label: 'Dashboard' },
+  { href: '/my/digest', label: 'Digest' },
   { href: '/my/groups', label: 'My Groups' },
   { href: '/my/settings', label: 'Settings' },
 ];