This PR does the following:

• provides cleaner handlers, by moving find/throw exeption logic to services
• removes duplicate checks across the code by having the above checks inside services
• find* service methods will do one of the following:
  • find the requested entity and return it, allowing the request to continue processing
  • not find the requested entity, throw a NotFoundException, which will pass through the handler and get caught by the HandlerTrait's handle method that returns a 404 Not Found response
• replaces old /** @ var Class $variable */ annotations with asserts, helping both the IDE and Psalm on inferring types