feat: article search by keyword

[hanw39]

Summary

Support filtering articles by keyword so users can quickly find relevant content without scrolling through the full list.

Proposed CLI

blogwatcher articles --search <keyword>

Can be combined with existing flags:

blogwatcher articles --search "golang" --blog "Paul Graham" --all

Behavior

• Match against article title (and URL as fallback)
• Case-insensitive
• Works alongside existing --blog and --all filters

Notes

• Search can be implemented at the SQL layer (LIKE '%keyword%') for efficiency