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.

internal/database/memory.go

@@ -213,7 +213,7 @@ func (db *MemoryDB) matchesFilter(entry *apiv0.ServerJSON, filter *ServerFilter)
 
 	// Check exact version filter
 	if filter.Version != nil {
-		if entry.VersionDetail.Version != *filter.Version {
+		if entry.Version != *filter.Version {
 			return false
 		}
 	}