Refactor: Transfer responsibility from handlers to services.

[alexmerlin]

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

[github-actions]

Qodana for PHP

It seems all right 👌

No new problems were found according to the checks applied

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