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.

scripts/test_publish.sh

@@ -86,9 +86,7 @@ cat > "$PAYLOAD_FILE" << EOF
     "source": "github",
     "id": "example/test-mcp-server"
   },
-  "version_detail": {
-    "version": "1.0.$(date +%s)"
-  },
+  "version": "1.0.$(date +%s)",
   "packages": [
     {
       "package_type": "javascript",