Add a built-in newsletter system powered by roundups, targeted by persona/section

[rvanmaanen]

Build a newsletter feature that leverages the existing roundup content:

• Allow users to subscribe to newsletters for specific sections/topics (e.g., "AI weekly", "GitHub Copilot updates").
• Automatically generate newsletter content from per-section roundups (see issue Implement explicit Azure Static Web App deployment trigger for RSS workflow #2).
• Email delivery integration (e.g., Azure Communication Services, SendGrid).
• Subscription management UI (subscribe/unsubscribe, preference center).
• Consider persona-based targeting: developer, team lead, architect, etc.