Clean up schema by simplifying version structure (#379)

## Summary

Simplified the schema structure by:
- Replacing `version_detail.version` with direct `version` field  
- Removing unused `release_date` field from seed data
- Updating all API types, models, and tests to match new structure
- Maintaining backwards compatibility in API responses

This change makes the schema more intuitive and reduces nesting depth.

docs/guides/publishing/github-actions.md

@@ -254,7 +254,7 @@ Keep your package version and server.json version in sync:
 - name: Update server.json version
   run: |
     VERSION=${GITHUB_REF#refs/tags/v}
-    jq --arg version "$VERSION" '.version_detail.version = $version' server.json > tmp.json
+    jq --arg version "$VERSION" '.version = $version' server.json > tmp.json
     mv tmp.json server.json
 ```