docs: review privacy and public data exposure

[DevSecNinja]

Why

Wazzup publishes generated data through GitHub Pages. That is fine for the current personal setup, but the docs should make the privacy and data exposure trade-offs explicit.

Scope

• Document what data is public: sources, interests, generated summaries, source status, and retained briefing history.
• Confirm that secrets, prompts with private material, full article text, and local-only preferences are not published.
• Identify optional private alternatives if the project ever needs them.
• Add a small checklist for future features that may affect data exposure.

Acceptance criteria

• The public/private data boundary is documented.
• Future contributors can tell what must not be written to public/data.
• Any recommended mitigations are lightweight and aligned with the GitHub-native architecture.