Replace Generator objects with classes

[milosmns]

Provide a brief summary of the planned work

All Generator objects should be replaced with classes we can instantiate and inject using Spring's DI.

Detailed overview

For historical purposes, generators were created as objects. Now that everything is in one codebase, there's no need for this legacy approach — we can migrate these objects to classes we can instantiate and use more easily through Dependency Injection (and in tests).

Hint/location:

com.appifyhub.monolith.features.user.repository.util.SignupCodeGenerator
com.appifyhub.monolith.features.user.repository.util.UserIdGenerator
com.appifyhub.monolith.features.user.repository.util.TokenGenerator
com.appifyhub.monolith.features.creator.repository.SignatureGenerator

List the acceptance criteria

• All mentioned generators are classes instead of objects
• All tests are updated to reflect this change, and passing

Include any additional notes (optional)

No response