Chore: Add all TODO backend models to the model repository

[AnujChhikara]

Description

We need to establish a well-defined data contract for all TODO-related entities in the system.

• We need to document the data structure of the todo-app database in the models repository, so that all services have a reference point for the TODO-related entities.
• Ensure both MongoDB and PostgreSQL structures are covered.

TODO

Document the data structure of every module/entity.

Define and list the schema for both MongoDB and PostgreSQL implementations.

[iamitprakash]

@AnujChhikara looking at the code and issues ticket desc, I am still not sure about this issues ticket like even this qualify or even needed.