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.

cmd/publisher/commands/init.go

@@ -309,9 +309,7 @@ func createServerJSON(
 			URL:    repoURL,
 			Source: repoSource,
 		},
-		VersionDetail: model.VersionDetail{
-			Version: version,
-		},
+		Version: version,
 		Packages: []model.Package{pkg},
 	}
 }