[Feature] RSS and Atom feed endpoints for DailyPaper

[jerry609]

Source TODO

• GET /api/feed/rss
• GET /api/feed/atom
• 输出静态 RSS 文件

Scope

• Feed generation service + endpoints
• Optional static reports/feed.xml

Acceptance Criteria

• Valid RSS 2.0 and Atom 1.0 output
• Supports recent-N and optional filters

[jerry609]

Implemented in commit 5c39aab on branch feat/dailypaper-sse-stream: new /api/feed/rss and /api/feed/atom endpoints with days/query/track filters and optional static feed file output.