Better blog post validation and exports

We’ve made blog post imports and exports more comprehensive with better validation and metafield support.

🔧 Improvements

• Published date validation – catches invalid dates before they cause import errors, including dates before 1970 and unrealistic future dates
• Blog post sheet recognition – now recognizes “Article” as a valid sheet name for blog post imports
• Metafield exports – blog post exports now include SEO metafields (title_tag and description_tag) plus any custom metafields you’ve defined

🐛 Fixes

• Handle matching – improved case-insensitive matching for blog post handles during imports