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.

pkg/model/types.go

@@ -90,8 +90,3 @@ type Argument struct {
 	IsRepeated         bool         `json:"is_repeated,omitempty"`
 	ValueHint          string       `json:"value_hint,omitempty"`
 }
-
-// VersionDetail represents the version details of a server (pure MCP spec, no registry metadata)
-type VersionDetail struct {
-	Version string `json:"version" minLength:"1"`
-}