Add CNN as a first-class news source with RSS fallback configuration

[Copilot]

New Feature

What does this feature do?

This change adds CNN to the app’s default source set and wires CNN into the existing source-specific RSS fetch fallback flow. CNN articles now participate in the same ingestion path as other built-in publishers.

Why is this feature needed?

The issue requires CNN to be available out of the box and fetchable using the same implementation pattern as existing sources (default source registration + per-source feed fallback handling).

Implementation details

• Default source registration
  • Added CNN to defaultSources in src/App.tsx so new installs/users see CNN as an available active source.
• Source-specific feed configuration
  • Extended fetchArticlesFromRSS in src/lib/articleService.ts with CNN alternate feed URLs, matching the existing per-source fallback pattern used for other providers.
• No schema/UI shape changes
  • Existing NewsSource model and source management UI already support arbitrary sources; no new model fields or UI components were required.

// src/App.tsx (default sources)
{
  id: "7",
  name: "CNN",
  url: "https://rss.cnn.com/rss/edition.rss",
  isActive: true
}

// src/lib/articleService.ts (source-specific fallback URLs)
} else if (source.name === "CNN") {
  possibleUrls.push(
    "https://rss.cnn.com/rss/edition_world.rss",
    "https://rss.cnn.com/rss/cnn_latest.rss"
  )
}