refac: register only open while no verified acc exists, enforced by a use case rather than handler & sends verification email whose link uses the conf pub URL whose code expires after 2h

api/pkg/apperr/messages.go

@@ -5,3 +5,5 @@ const BodyErrorMessage = "The request cannot be processed due to a mismatch in t
 const IdUint32ErrorMessage = "The server expects the ID to be in the format of an unsigned 32-bit integer (uint32)."
 const ArticleNotFoundErrorMessage = "Article not found"
 const ArticleTitleErrorMessage = "Article title must be between 3 and 200 characters."
+const RegistrationClosedErrorMessage = "You are not allowed to create a user. Log in or reset database."
+const RegistrationFailedErrorMessage = "Registration failed. Please try again later."